/*
 * very simple handwritten recursive descent
 * parser for a catskill source file.
 */

#define PARSER_LOOKAHEAD 2

#define CHECK(parse) \
    parse;           \
    if (!parser_error_is_none(error)) return nil;

#define CHECK_RETURN(parse, ret) \
    parse;                       \
    if (!parser_error_is_none(error)) return (ret){ 0 };

struct Parser_Error
{
    enum Parser_Error_Kind
    {
        PARSER_ERROR_NONE,
        PARSER_ERROR_UNEXPECTED_TOKEN,
        PARSER_ERROR_UNEXPECTED_EOF,
        PARSER_ERROR_EXPECTED_STATEMENT_END,
        PARSER_ERROR_EXPECTED_PRIMARY_EXPRESSION,
        PARSER_ERROR_EXPECTED_TYPE,
    } kind;
    // TODO: add span to error
};

struct Parser_Error
parser_error(enum Parser_Error_Kind kind)
{
    return (struct Parser_Error){ kind };
}

struct Parser_Error
parser_error_none(void)
{
    return parser_error(PARSER_ERROR_NONE);
}

bool
parser_error_is_none(const struct Parser_Error* error)
{
    return error->kind == PARSER_ERROR_NONE;
}

const ascii*
parser_error_to_string(const struct Parser_Error* error)
{
    switch (error->kind) {
    case PARSER_ERROR_NONE:
        return "none";
    case PARSER_ERROR_UNEXPECTED_TOKEN:
        return "unexpected token";
    case PARSER_ERROR_UNEXPECTED_EOF:
        return "unexpected end of file";
    case PARSER_ERROR_EXPECTED_STATEMENT_END:
        return "expected statement end";
    case PARSER_ERROR_EXPECTED_PRIMARY_EXPRESSION:
        return "expected primary expression";
    case PARSER_ERROR_EXPECTED_TYPE:
        return "expected type";
    default:
        return "unknown error";
    }
}

struct Parser
{
    struct Lexer* lexer;
    struct Token lookahead[PARSER_LOOKAHEAD];
};

void
parser_new(struct Parser* p, struct Lexer* lexer)
{
    p->lexer = lexer;
    memset(p->lookahead, 0, sizeof(p->lookahead));
}

bool
parser_reached_end(struct Parser* p)
{
    return p->lexer->eof;
}

bool
parser_lookahead_pop(struct Parser* p, struct Token* token)
{
    struct Token head = p->lookahead[0];
    if (token_is_empty(&head)) return false;

    for (uint i = 0; i < PARSER_LOOKAHEAD - 1; i++) p->lookahead[i] = p->lookahead[i + 1];
    p->lookahead[PARSER_LOOKAHEAD - 1] = token_none();

    *token = head;
    return true;
}

bool
parser_lookahead_push(struct Parser* p, struct Token token)
{
    for (uint i = 0; i < PARSER_LOOKAHEAD; i++) {
        if (token_is_empty(&p->lookahead[i])) {
            p->lookahead[i] = token;
            return true;
        }
    }
    return false;
}

// advance the token stream and return the next token.
struct Token
parser_next(struct Parser* p)
{
    struct Token token;
    if (!parser_lookahead_pop(p, &token)) token = lexer_next(p->lexer);
    return token;
}

// advance the token stream if the token is of the right type.
// if not, return none-token and set the error.
struct Token
parser_need(struct Parser* p, enum Token_Kind kind, struct Parser_Error* error)
{
    struct Token token = parser_next(p);
    if (!token_is(&token, kind)) {
        *error = parser_error(PARSER_ERROR_UNEXPECTED_TOKEN);
        return token_none();
    }
    return token;
}

// peek at the token stream at given index, without advancing it.
// index 0 is the token that would be returned by a next `parser_next` call.
// peek limit is defined by `PARSER_LOOKAHEAD`.
struct Token
parser_peek_at(struct Parser* p, uint index)
{
    check(index < 2, "parser peek index out of range");

    while (token_is_empty(&p->lookahead[index])) {
        struct Token next_token = lexer_next(p->lexer);
        parser_lookahead_push(p, next_token);
    }

    return p->lookahead[index];
}

// peek at the next token in the stream, without advancing it.
// synonym for `parser_peek_at(p, 0)`.
struct Token
parser_peek(struct Parser* p)
{
    return parser_peek_at(p, 0);
}

// peek at the token one beyond the current cursor in the stream, without advancing it.
// synonym for `parser_peek_at(p, 1)`.
struct Token
parser_peek_further(struct Parser* p)
{
    return parser_peek_at(p, 1);
}

// check if the next token is of a given type.
bool
parser_probe(struct Parser* p, enum Token_Kind kind)
{
    struct Token token = parser_peek(p);
    return token_is(&token, kind);
}

// skip all consecutive newlines and other non-interrupting
// tokens in the token stream.
// necessary for parsing statements and expressions that are
// split-able over multiple lines.
// TODO: this needs to be used in many more places and
// is currently mostly redundant, due to only skipping newlines,
// acting mostly as a marking for `the tokens don't have to follow precisely`.
void
parser_unglue(struct Parser* p)
{
    while (parser_probe(p, TOKEN_NEWLINE)) parser_next(p);
}

struct Statement* parser_statement(struct Parser* p, struct Parser_Error* error);
struct Expression* parser_expression(struct Parser* p, struct Parser_Error* error);

void
parser_end_statement(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = parser_peek(p);
    if (!token_ends_statement(&token)) {
        *error = parser_error(PARSER_ERROR_EXPECTED_STATEMENT_END);
        return;
    }
    parser_next(p);
}

struct Block_Node
parser_block_node(struct Parser* p, struct Parser_Error* error)
{
    struct Token start_token =
        CHECK_RETURN(parser_need(p, TOKEN_CURLY_OPEN, error), struct Block_Node);

    parser_unglue(p);

    struct Statement* head = nil;
    struct Statement* current = nil;

    while (!parser_probe(p, TOKEN_CURLY_CLOSE)) {
        struct Statement* statement = CHECK_RETURN(parser_statement(p, error), struct Block_Node);
        CHECK_RETURN(parser_end_statement(p, error), struct Block_Node);

        if (!head) {
            head = statement;
        } else {
            current->next = statement;
        }
        current = statement;
    }

    struct Token end_token =
        CHECK_RETURN(parser_need(p, TOKEN_CURLY_CLOSE, error), struct Block_Node);
    struct Span span = span_merge(start_token.span, end_token.span);

    return (struct Block_Node){
        .statements = head,
        .span = span,
        .location = start_token.location,
    };
}

struct Type_Node* parser_node_type(struct Parser* p, struct Parser_Error* error);

struct Function_Header_Node
parser_function_header_node(struct Parser* p, struct Parser_Error* error)
{
    struct Token open_parameters_token =
        CHECK_RETURN(parser_need(p, TOKEN_ROUND_OPEN, error), struct Function_Header_Node);

    struct Function_Header_Node header = { 0 };
    while (!parser_probe(p, TOKEN_ROUND_CLOSE)) {
        struct Token name_token =
            CHECK_RETURN(parser_need(p, TOKEN_NAME, error), struct Function_Header_Node);
        struct String name = name_token.value.name;

        struct Type_Node* type;
        if (!parser_probe(p, TOKEN_ROUND_CLOSE) && !parser_probe(p, TOKEN_COMMA)) {
            type = CHECK_RETURN(parser_node_type(p, error), struct Function_Header_Node);
        } else {
            type = type_node_none(name_token.span, name_token.location);
        }

        type->value_name = name;
        if (!header.parameters_type_and_name)
            header.parameters_type_and_name = type;
        else
            header.parameters_type_and_name->next = type;

        if (parser_probe(p, TOKEN_COMMA)) parser_next(p);
    }
    struct Token close_parameters_token = parser_next(p);

    header.span = span_merge(open_parameters_token.span, close_parameters_token.span);

    struct Token next = parser_peek(p);
    struct Token further = parser_peek_further(p);
    if (token_can_begin_type(&next)) {
        // NOTE: this is very uncomfortable. to avoid ambiguity between a no-return-type function
        // body (i.e. `fun () {`) and a function with a return type of inline structure (i.e. `fun
        // () {x int} {`), we require that inline structs are declared in one line, without a
        // line-break, and function bodies always have a line-break after the opening brace. this
        // means that we cannot have a single line function, and we can't have a function with an
        // inline structure as a return type that is too long to fit on a single line.
        // TODO: to allow single line functions, introduce a `:` token as a replacement for the
        // opening brace. `fun (x int): x + 1`
        if (token_is(&next, TOKEN_CURLY_OPEN) && token_is(&further, TOKEN_NEWLINE)) return header;

        header.return_type = CHECK_RETURN(parser_node_type(p, error), struct Function_Header_Node);
        header.span = span_merge(header.span, header.return_type->span);
    }

    return header;
}

struct Bare_Declaration_Node
parser_bare_declaration_node(struct Parser* p, struct Parser_Error* error)
{
    struct String_Array names = string_array_new();

    struct Span span = { 0 };
    struct Cursor location = parser_peek(p).location;
    for (;;) {
        struct Token name_token =
            CHECK_RETURN(parser_need(p, TOKEN_NAME, error), struct Bare_Declaration_Node);

        span = span_is_empty(span) ? name_token.span : span_merge(span, name_token.span);
        string_array_add(&names, name_token.value.name);

        struct Token next = parser_peek(p);
        if (token_can_begin_type(&next)) break;
        if (next.kind == TOKEN_COMMA) parser_next(p);
    }

    // for now, type is always required.
    struct Type_Node* type = CHECK_RETURN(parser_node_type(p, error), struct Bare_Declaration_Node);
    CHECK_RETURN(parser_need(p, TOKEN_ASSIGN, error), struct Bare_Declaration_Node);
    struct Expression* initializer =
        CHECK_RETURN(parser_expression(p, error), struct Bare_Declaration_Node);

    return (struct Bare_Declaration_Node){
        .names = names,
        .initializer = initializer,
        .type = type,

        .span = span,
        .location = location,
    };
}

struct Type_Node*
parser_node_type_name(struct Parser* p, struct Parser_Error* error)
{
    struct Token name_token = CHECK(parser_need(p, TOKEN_NAME, error));
    return type_node_new(
        TYPE_NODE_NAME, (union Type_Node_Value){ .name = { name_token.value.name } },
        name_token.span, name_token.location);
}

struct Type_Node*
parser_node_type_structure(struct Parser* p, struct Parser_Error* error)
{
    struct Token open_token = CHECK(parser_need(p, TOKEN_CURLY_OPEN, error));

    parser_unglue(p);

    struct Type_Node *head = nil, *current = nil;
    while (!parser_probe(p, TOKEN_CURLY_CLOSE)) {
        struct Token field_name_token = CHECK(parser_need(p, TOKEN_NAME, error));

        struct Type_Node* field_type = CHECK(parser_node_type(p, error));
        field_type->value_name = field_name_token.value.name;

        if (!head)
            head = field_type;
        else
            current->next = field_type;
        current = field_type;

        if (parser_probe(p, TOKEN_COMMA) || parser_probe(p, TOKEN_NEWLINE)) parser_next(p);
    }

    parser_unglue(p);

    struct Token close_token = CHECK(parser_need(p, TOKEN_CURLY_CLOSE, error));
    struct Span span = span_merge(open_token.span, close_token.span);

    return type_node_new(
        TYPE_NODE_STRUCTURE, (union Type_Node_Value){ .structure = { head } }, span,
        open_token.location);
}

struct Type_Node*
parser_node_type_variant(struct Parser* p, struct Parser_Error* error)
{
    struct Token variant_token = CHECK(parser_need(p, TOKEN_WORD_VARIANT, error));
    CHECK(parser_need(p, TOKEN_CURLY_OPEN, error));

    parser_unglue(p);

    struct Type_Node *head = nil, *current = nil;
    while (!parser_probe(p, TOKEN_CURLY_CLOSE)) {
        struct Token variant_name_token = CHECK(parser_need(p, TOKEN_NAME, error));
        struct String variant_name = variant_name_token.value.name;

        struct Type_Node* backing_type = nil;

        struct Token next = parser_peek(p);
        bool has_backing_type =
            !token_is(&next, TOKEN_NEWLINE) && !token_is(&next, TOKEN_COMMA)
            && !token_is(&next, TOKEN_CURLY_CLOSE);

        if (has_backing_type) {
            backing_type = CHECK(parser_node_type(p, error));
        } else {
            backing_type = type_node_none(variant_name_token.span, variant_name_token.location);
        }

        backing_type->value_name = variant_name;
        if (!head)
            head = backing_type;
        else
            current->next = backing_type;
        current = backing_type;

        next = parser_peek(p);
        if (token_is(&next, TOKEN_COMMA) || token_is(&next, TOKEN_NEWLINE)) parser_next(p);
    }

    parser_unglue(p);

    struct Token close_token = CHECK(parser_need(p, TOKEN_CURLY_CLOSE, error));
    struct Span span = span_merge(variant_token.span, close_token.span);

    return type_node_new(
        TYPE_NODE_VARIANT, (union Type_Node_Value){ .variant = { head } }, span,
        variant_token.location);
}

struct Type_Node*
parser_node_type_function(struct Parser* p, struct Parser_Error* error)
{
    struct Token fun_token = CHECK(parser_need(p, TOKEN_WORD_FUN, error));
    struct Function_Header_Node header = CHECK(parser_function_header_node(p, error));

    struct Span span = span_merge(fun_token.span, header.span);
    return type_node_new(
        TYPE_NODE_FUNCTION, (union Type_Node_Value){ .function = { header } }, span,
        fun_token.location);
}

struct Type_Node*
parser_node_type_class(struct Parser* p, struct Parser_Error* error)
{
    struct Token class_token = CHECK(parser_need(p, TOKEN_WORD_CLASS, error));
    CHECK(parser_need(p, TOKEN_CURLY_OPEN, error));

    parser_unglue(p);

    struct Type_Node *head = nil, *current = nil;
    while (!parser_probe(p, TOKEN_CURLY_CLOSE)) {
        struct Token method_name_token = CHECK(parser_need(p, TOKEN_NAME, error));
        struct String method_name = method_name_token.value.name;

        struct Function_Header_Node header = CHECK(parser_function_header_node(p, error));
        // allocate a new type node for the method type,
        // for the sake of consistency and the built-in type node linked list.
        struct Type_Node* method_type = type_node_new(
            TYPE_NODE_FUNCTION, (union Type_Node_Value){ .function = { header } }, header.span,
            header.parameters_type_and_name->location);
        method_type->value_name = method_name;

        if (!head)
            head = method_type;
        else
            current->next = method_type;
        current = method_type;

        if (parser_probe(p, TOKEN_COMMA) || parser_probe(p, TOKEN_NEWLINE)) parser_next(p);
    }

    parser_unglue(p);

    struct Token close_token = CHECK(parser_need(p, TOKEN_CURLY_CLOSE, error));
    struct Span span = span_merge(class_token.span, close_token.span);
    return type_node_new(
        TYPE_NODE_CLASS, (union Type_Node_Value){ .class = { head } }, span, class_token.location);
}

struct Type_Node*
parser_node_type_tuple(struct Parser* p, struct Parser_Error* error)
{
    struct Token open_token = CHECK(parser_need(p, TOKEN_ROUND_OPEN, error));

    struct Type_Node *head = nil, *current = nil;
    while (!parser_probe(p, TOKEN_ROUND_CLOSE)) {
        struct Type_Node* type = CHECK(parser_node_type(p, error));

        if (!head)
            head = type;
        else
            current->next = type;
        current = type;

        if (parser_probe(p, TOKEN_COMMA))
            parser_next(p);
        else
            break;
    }

    struct Token close_token = CHECK(parser_need(p, TOKEN_ROUND_CLOSE, error));
    struct Span span = span_merge(open_token.span, close_token.span);
    return type_node_new(
        TYPE_NODE_TUPLE, (union Type_Node_Value){ .tuple = { head } }, span, open_token.location);
}

struct Type_Node*
parser_node_type_array_or_map(struct Parser* p, struct Parser_Error* error)
{
    struct Token open_token = CHECK(parser_need(p, TOKEN_SQUARE_OPEN, error));

    struct Type_Node* element_or_key_type = CHECK(parser_node_type(p, error));
    if (!element_or_key_type) {
        *error = parser_error(PARSER_ERROR_EXPECTED_TYPE);
        return nil;
    }

    enum Type_Node_Type type;
    union Type_Node_Value value;
    if (parser_probe(p, TOKEN_ASSIGN)) {
        // this is a map type, e.g. `[string = int]`
        parser_next(p); // consume the assignment token

        struct Type_Node* key_type = element_or_key_type;
        struct Type_Node* value_type = CHECK(parser_node_type(p, error));
        if (!value_type) {
            *error = parser_error(PARSER_ERROR_EXPECTED_TYPE);
            return nil;
        }

        type = TYPE_NODE_MAP;
        value.map = (struct Type_Node_Map){
            .key_type = key_type,
            .value_type = value_type,
        };
    } else {
        // this is an array type, e.g. `[int]`
        type = TYPE_NODE_ARRAY;
        value.array = (struct Type_Node_Array){ .element_type = element_or_key_type };
    }

    struct Token close_token = CHECK(parser_need(p, TOKEN_SQUARE_CLOSE, error));

    struct Span span = span_merge(open_token.span, close_token.span);
    return type_node_new(type, value, span, open_token.location);
}

struct Type_Node*
parser_node_type_reference(struct Parser* p, struct Parser_Error* error)
{
    struct Token ampersand_token = CHECK(parser_need(p, TOKEN_AMPERSAND, error));

    struct Type_Node* referenced_type = CHECK(parser_node_type(p, error));
    if (!referenced_type) {
        *error = parser_error(PARSER_ERROR_EXPECTED_TYPE);
        return nil;
    }

    struct Span span = span_merge(ampersand_token.span, referenced_type->span);
    return type_node_new(
        TYPE_NODE_REFERENCE, (union Type_Node_Value){ .reference = { referenced_type } }, span,
        ampersand_token.location);
}

struct Type_Node*
parser_node_type(struct Parser* p, struct Parser_Error* error)
{
    // TODO: maybe, variant, class
    struct Token token = parser_peek(p);
    switch (token.kind) {
    case TOKEN_NAME:
        return parser_node_type_name(p, error);
    case TOKEN_WORD_VARIANT:
        return parser_node_type_variant(p, error);
    case TOKEN_WORD_CLASS:
        return parser_node_type_class(p, error);
    case TOKEN_WORD_FUN:
        return parser_node_type_function(p, error);
    case TOKEN_CURLY_OPEN:
        return parser_node_type_structure(p, error);
    case TOKEN_ROUND_OPEN:
        return parser_node_type_tuple(p, error);
    case TOKEN_SQUARE_OPEN:
        return parser_node_type_array_or_map(p, error);
    case TOKEN_AMPERSAND:
        return parser_node_type_reference(p, error);
    default:
        *error = parser_error(PARSER_ERROR_EXPECTED_TYPE);
        return nil;
    }
}

struct Expression*
parser_expression_primary_name(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = CHECK(parser_need(p, TOKEN_NAME, error));
    union Expression_Value value = { .name = { token.value.name } };
    return expression_new(EXPRESSION_NAME, value, token.span, token.location);
}

struct Expression*
parser_expression_primary_integer(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = CHECK(parser_need(p, TOKEN_LITERAL_INTEGER, error));
    union Expression_Value value = { .integer_literal = { token.value.literal_integer } };
    return expression_new(EXPRESSION_INTEGER_LITERAL, value, token.span, token.location);
}

struct Expression*
parser_expression_primary_float(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = CHECK(parser_need(p, TOKEN_LITERAL_FLOAT, error));
    union Expression_Value value = { .float_literal = { token.value.literal_float } };
    return expression_new(EXPRESSION_FLOAT_LITERAL, value, token.span, token.location);
}

struct Expression*
parser_expression_primary_string(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = CHECK(parser_need(p, TOKEN_LITERAL_STRING, error));
    union Expression_Value value = { .string_literal = { token.value.literal_string } };
    return expression_new(EXPRESSION_STRING_LITERAL, value, token.span, token.location);
}

struct Expression*
parser_expression_primary_boolean(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = parser_next(p);
    check(token.kind == TOKEN_WORD_TRUE || token.kind == TOKEN_WORD_FALSE,
          "expected boolean literal");
    bool literal = token.kind == TOKEN_WORD_TRUE;
    union Expression_Value expr_value = { .bool_literal = { literal } };
    return expression_new(EXPRESSION_BOOLEAN_LITERAL, expr_value, token.span, token.location);
}

struct Expression*
parser_expression_primary_group(struct Parser* p, struct Parser_Error* error)
{
    struct Token start_token = CHECK(parser_need(p, TOKEN_ROUND_OPEN, error));
    struct Expression* expression = CHECK(parser_expression(p, error));
    struct Token end_token = CHECK(parser_need(p, TOKEN_ROUND_CLOSE, error));

    struct Span span = span_merge(start_token.span, end_token.span);
    union Expression_Value value = { .group = { expression } };
    return expression_new(EXPRESSION_GROUP, value, span, start_token.location);
}

struct Expression*
parser_expression_function(struct Parser* p, struct Parser_Error* error)
{
    struct Token fun_token = CHECK(parser_need(p, TOKEN_WORD_FUN, error));
    struct Expression_Function fun = { 0 };

    fun.header = CHECK(parser_function_header_node(p, error));
    fun.body = CHECK(parser_block_node(p, error));

    return expression_new(
        EXPRESSION_FUNCTION, (union Expression_Value){ .function = fun },
        span_merge(fun_token.span, fun.body.span), fun_token.location);
}

struct Expression*
parser_expression_type(struct Parser* p, struct Parser_Error* error)
{
    struct Token start_token = parser_peek(p);

    struct Type_Node* type = nil;
    switch (start_token.kind) {
    case TOKEN_WORD_TYPE:
        parser_next(p); // skip the `type` keyword.
        type = CHECK(parser_node_type(p, error));
        break;
    case TOKEN_WORD_VARIANT:
        type = CHECK(parser_node_type_variant(p, error));
        break;
    case TOKEN_WORD_CLASS:
        type = CHECK(parser_node_type_class(p, error));
        break;
    default:
        failure("expected type signifying keyword");
    }

    struct Span span = span_merge(start_token.span, type->span);
    return expression_new(
        EXPRESSION_TYPE, (union Expression_Value){ .type = { type } }, span, start_token.location);
}

struct Expression*
parser_expression_primary(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = parser_peek(p);
    switch (token.kind) {
    case TOKEN_NAME:
        return parser_expression_primary_name(p, error);
    case TOKEN_LITERAL_INTEGER:
        return parser_expression_primary_integer(p, error);
    case TOKEN_LITERAL_FLOAT:
        return parser_expression_primary_float(p, error);
    case TOKEN_LITERAL_STRING:
        return parser_expression_primary_string(p, error);
    case TOKEN_WORD_TRUE:
    case TOKEN_WORD_FALSE:
        return parser_expression_primary_boolean(p, error);
    case TOKEN_ROUND_OPEN:
        return parser_expression_primary_group(p, error);
    case TOKEN_WORD_FUN:
        return parser_expression_function(p, error);
    // `variant` and `class` here are essentially a shortcut, as they already
    // imply a type, so we don't have to write `type variant` or `type class`.
    case TOKEN_WORD_TYPE:
    case TOKEN_WORD_VARIANT:
    case TOKEN_WORD_CLASS:
        return parser_expression_type(p, error);
    default:
        *error = parser_error(PARSER_ERROR_EXPECTED_PRIMARY_EXPRESSION);
        return nil;
    }
}

struct Expression*
parser_expression_member(struct Parser* p, struct Parser_Error* error)
{
    struct Expression* left = CHECK(parser_expression_primary(p, error));

    // NOTE: see `parser_expression_postfix_call`.
    while (parser_probe(p, TOKEN_DOT)) {
        parser_next(p);
        struct Token name_token = CHECK(parser_need(p, TOKEN_NAME, error));
        struct String name = name_token.value.name;

        struct Span span = span_merge(left->span, name_token.span);
        union Expression_Value value = { .member = { left, name } };
        left = expression_new(EXPRESSION_MEMBER, value, span, name_token.location);
    }

    return left;
}

struct Expression*
parser_expression_postfix_call(
    struct Parser* p, struct Expression* subject, struct Parser_Error* error)
{
    // NOTE: because of the way the parser works, we have to parse all subsequent
    // call expressions in the same loop.
    // this is the case with an expression like `meow_function()(123)`,
    // where the hypothetical `meow_function` returns a function pointer.
    while (parser_probe(p, TOKEN_ROUND_OPEN)) {
        parser_next(p);

        struct Expression *arguments_head = nil, *arguments_current = nil;
        while (!parser_probe(p, TOKEN_ROUND_CLOSE)) {
            struct Expression* argument = CHECK(parser_expression(p, error));
            if (!arguments_head)
                arguments_head = argument;
            else
                arguments_current->next = argument;
            arguments_current = argument;
            if (parser_probe(p, TOKEN_COMMA)) parser_next(p);
        }

        struct Token token = CHECK(parser_need(p, TOKEN_ROUND_CLOSE, error));
        struct Span span = span_merge(subject->span, token.span);
        union Expression_Value value = { .call = { subject, arguments_head } };
        subject = expression_new(EXPRESSION_CALL, value, span, token.location);
    }

    return subject;
}

struct Expression*
parser_expression_postfix_subscript(
    struct Parser* p, struct Expression* subject, struct Parser_Error* error)
{
    // NOTE: see `parser_expression_postfix_call`.
    while (parser_probe(p, TOKEN_SQUARE_OPEN)) {
        parser_next(p);

        struct Expression* index = CHECK(parser_expression(p, error));
        struct Token token = CHECK(parser_need(p, TOKEN_SQUARE_CLOSE, error));

        struct Span span = span_merge(subject->span, span_merge(index->span, token.span));
        union Expression_Value value = { .subscript = { subject, index } };
        subject = expression_new(EXPRESSION_SUBSCRIPT, value, span, token.location);
    }
    return subject;
}

struct Expression*
parser_expression_postfix(struct Parser* p, struct Parser_Error* error)
{
    struct Expression* expression = CHECK(parser_expression_member(p, error));

    struct Token token = parser_peek(p);
    switch (token.kind) {
    case TOKEN_ROUND_OPEN:
        return parser_expression_postfix_call(p, expression, error);
    case TOKEN_SQUARE_OPEN:
        return parser_expression_postfix_subscript(p, expression, error);
    default:;
    }

    enum Increment_Decrement_Operation inc_dec_op =
        increment_decrement_operation_from_token(&token);
    if (inc_dec_op) {
        parser_next(p);
        struct Expression_Increment_Decrement inc_dec = {
            .prefix = false,
            .subject = expression,
            .operation = inc_dec_op,
        };

        struct Span span = span_merge(expression->span, token.span);
        union Expression_Value value = { .increment_decrement = inc_dec };
        return expression_new(EXPRESSION_INCREMENT_DECREMENT, value, span, token.location);
    }

    return expression;
}

struct Expression*
parser_expression_unary_operation(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = parser_peek(p);
    enum Unary_Operation operation = unary_operation_from_token(&token);
    if (operation) {
        parser_next(p);
        struct Expression* operand = CHECK(parser_expression_unary_operation(p, error));

        struct Span span = span_merge(token.span, operand->span);
        union Expression_Value value = { .unary_operator = { operation, operand } };
        return expression_new(EXPRESSION_UNARY_OPERATION, value, span, token.location);
    }

    enum Increment_Decrement_Operation inc_dec_op =
        increment_decrement_operation_from_token(&token);
    if (inc_dec_op) {
        parser_next(p);
        struct Expression* subject = CHECK(parser_expression_unary_operation(p, error));
        struct Expression_Increment_Decrement inc_dec = {
            .prefix = true,
            .subject = subject,
            .operation = inc_dec_op,
        };

        struct Span span = span_merge(token.span, inc_dec.subject->span);
        union Expression_Value value = { .increment_decrement = inc_dec };
        return expression_new(EXPRESSION_INCREMENT_DECREMENT, value, span, token.location);
    }

    return parser_expression_postfix(p, error);
}

// given two expressions and some kind of binary operation between them,
// merge them into a single binary expression, with attention to operator precedence
// and associativity.
struct Expression_Binary_Operator
parser_merge_into_single_binary_expression(
    struct Parser* p, struct Expression* left, enum Binary_Operation operation,
    struct Expression* right)
{
    // NOTE: due to the parser structure, the left expression is never a binary operation
    // so we can worry about fixing up the right side only.
    check(left->kind != EXPRESSION_BINARY_OPERATION, "left expression is a binary operation");
    if (right->kind == EXPRESSION_BINARY_OPERATION) {
        struct Expression_Binary_Operator right_binary = right->value.binary_operator;

        uint right_precedence = binary_operation_precedence(right_binary.operation);
        uint precedence = binary_operation_precedence(operation);

        bool switch_due_to_precedence = right_precedence < precedence;
        bool switch_due_to_associativity =
            precedence == right_precedence
            && binary_operation_associativity(operation) == BINARY_ASSOCIATIVITY_LEFT;

        // essentially check if an expression like `a ~ (b ~ c)` needs to be
        // switched into `(a ~ b) ~ c` or not.
        if (switch_due_to_precedence || switch_due_to_associativity) {
            // since we only use static memory, we need to switch the operands around
            // without allocating more expressions, thus we have to reuse the allocation
            // slots of the previous expressions.
            struct Expression* operands[3] = {
                left, right_binary.left_operand, right_binary.right_operand
            };
            struct Expression* slots[2] = { right, right_binary.right_operand };

            struct Expression_Binary_Operator new_binary_operator =
                parser_merge_into_single_binary_expression(p, operands[0], operation, operands[1]);
            *slots[0] = (struct Expression){
                .kind = EXPRESSION_BINARY_OPERATION,
                .value = { .binary_operator = new_binary_operator },
                .span = span_merge(left->span, right->span),
                .location = left->location
            };

            *slots[1] = *operands[2];

            left = slots[0];
            right = slots[1];
            operation = right_binary.operation;
        }
    }

    return (struct Expression_Binary_Operator){
        .operation = operation,
        .left_operand = left,
        .right_operand = right,
    };
}

struct Expression*
parser_expression_binary_operation(struct Parser* p, struct Parser_Error* error)
{
    struct Expression* left = CHECK(parser_expression_unary_operation(p, error));

    struct Token token = parser_peek(p);
    enum Binary_Operation operation = binary_operation_from_token(&token);
    if (operation) {
        parser_next(p);
        struct Expression* right = CHECK(parser_expression_binary_operation(p, error));

        struct Span span = span_merge(left->span, right->span);
        struct Expression_Binary_Operator binary_value =
            parser_merge_into_single_binary_expression(p, left, operation, right);
        union Expression_Value value = { .binary_operator = binary_value };
        return expression_new(EXPRESSION_BINARY_OPERATION, value, span, left->location);
    }

    return left;
}

struct Expression*
parser_expression(struct Parser* p, struct Parser_Error* error)
{
    return parser_expression_binary_operation(p, error);
}

struct Statement*
parser_statement_declaration(struct Parser* p, struct Parser_Error* error)
{
    struct Token declaration_token = parser_next(p);

    enum Statement_Declaration_Kind declaration_kind =
        statement_declaration_kind_from_token(&declaration_token);
    check(declaration_kind, "expected valid declaration token");

    struct Bare_Declaration_Node inner = CHECK(parser_bare_declaration_node(p, error));

    struct Span span = span_merge(declaration_token.span, inner.span);
    union Statement_Value value = {
        .declaration = {
            .kind = declaration_kind,
            .inner = inner,
        },
    };
    return statement_new(STATEMENT_DECLARATION, value, span, declaration_token.location);
}

struct Statement*
parser_statement_conditional(struct Parser* p, struct Parser_Error* error)
{
    struct Statement_Value_Conditional conditional = { 0 };
    struct Token if_token = parser_need(p, TOKEN_WORD_IF, error);

    // primary if condition + block.
    struct Expression* if_condition = CHECK(parser_expression(p, error));
    struct Block_Node then_block = CHECK(parser_block_node(p, error));
    conditional.conditions[conditional.condition_count++] = (struct Statement_Conditional_Branch){
        .when = if_condition,
        .then = then_block,
    };

    struct Span span = span_merge(if_token.span, then_block.span);
    while (parser_probe(p, TOKEN_WORD_ELSE)) {
        check(conditional.condition_count < STATEMENT_VALUE_CONDITIONAL_MAX,
              "too many conditional branches");
        parser_next(p);

        struct Statement_Conditional_Branch branch = { 0 };
        if (parser_probe(p, TOKEN_WORD_IF)) {
            // else if condition + block.
            parser_next(p);
            struct Expression* else_condition = CHECK(parser_expression(p, error));
            struct Block_Node else_block = CHECK(parser_block_node(p, error));

            branch = (struct Statement_Conditional_Branch){
                .when = else_condition,
                .then = else_block,
            };
        } else {
            // else block.
            struct Block_Node else_block = CHECK(parser_block_node(p, error));
            branch = (struct Statement_Conditional_Branch){
                .when = nil,
                .then = else_block,
            };
        }

        conditional.conditions[conditional.condition_count++] = branch;
        span = span_merge(span, branch.then.span);
    }

    return statement_new(
        STATEMENT_CONDITIONAL, (union Statement_Value){ .conditional = conditional }, span,
        if_token.location);
}

struct Statement*
parser_statement_for(struct Parser* p, struct Parser_Error* error)
{
    struct Token for_token = CHECK(parser_need(p, TOKEN_WORD_FOR, error));

    // these are the possible for loop variants:
    // * `for name String = collection {}`, as iteration over container
    // * `for i u8 = 0, i < 10, i++ {}`, as a c-style semi-semi loop

    // a declaration without a signifier like `var` or `let`.
    struct Bare_Declaration_Node declaration = CHECK(parser_bare_declaration_node(p, error));
    enum Statement_Loop_Style style = STATEMENT_LOOP_STYLE_FOR_EACH;

    // c-style semi-semi loop.
    struct Expression *condition = nil, *iteration = nil;
    if (parser_probe(p, TOKEN_COMMA)) {
        parser_next(p);
        condition = CHECK(parser_expression(p, error));
        CHECK(parser_need(p, TOKEN_COMMA, error));
        iteration = CHECK(parser_expression(p, error));

        style = STATEMENT_LOOP_STYLE_C;
    }

    struct Block_Node body = CHECK(parser_block_node(p, error));

    struct Span span = span_merge(for_token.span, body.span);
    union Statement_Value value = {
        .loop = {
            .style = style,
            .declaration = declaration,
            .condition = condition,
            .iteration = iteration,
            .body = body,
        },
    };
    return statement_new(STATEMENT_LOOP, value, span, for_token.location);
}

struct Statement*
parser_statement_while(struct Parser* p, struct Parser_Error* error)
{
    struct Token while_token = CHECK(parser_need(p, TOKEN_WORD_WHILE, error));

    enum Statement_Loop_Style style = STATEMENT_LOOP_STYLE_ENDLESS;
    struct Expression* condition = nil;
    if (!parser_probe(p, TOKEN_CURLY_OPEN)) {
        condition = CHECK(parser_expression(p, error));
        style = STATEMENT_LOOP_STYLE_WHILE;
    }

    struct Block_Node body = CHECK(parser_block_node(p, error));

    struct Span span = span_merge(while_token.span, body.span);
    union Statement_Value value = {
        .loop = { .style = style, .condition = condition, .body = body }
    };
    return statement_new(STATEMENT_LOOP, value, span, while_token.location);
}

struct Statement*
parser_statement_block(struct Parser* p, struct Parser_Error* error)
{
    struct Block_Node block = CHECK(parser_block_node(p, error));
    return statement_new(
        STATEMENT_BLOCK, (union Statement_Value){ .block = { block } }, block.span, block.location);
}

struct Statement*
parser_statement_return(struct Parser* p, struct Parser_Error* error)
{
    struct Token return_token = CHECK(parser_need(p, TOKEN_WORD_RETURN, error));

    struct Expression* value = nil;
    if (!token_ends_statement(&return_token)) { value = CHECK(parser_expression(p, error)); }

    struct Span span = value ? return_token.span : span_merge(return_token.span, value->span);
    union Statement_Value statement_value = { .return_value = { value } };
    return statement_new(STATEMENT_RETURN, statement_value, span, return_token.location);
}

struct Statement*
parser_statement_break(struct Parser* p, struct Parser_Error* error)
{
    struct Token break_token = CHECK(parser_need(p, TOKEN_WORD_BREAK, error));
    struct Span span = break_token.span;
    return statement_new(STATEMENT_BREAK, (union Statement_Value){ 0 }, span, break_token.location);
}

struct Statement*
parser_statement_continue(struct Parser* p, struct Parser_Error* error)
{
    struct Token continue_token = CHECK(parser_need(p, TOKEN_WORD_CONTINUE, error));
    struct Span span = continue_token.span;
    return statement_new(
        STATEMENT_CONTINUE, (union Statement_Value){ 0 }, span, continue_token.location);
}

struct Statement*
parser_statement_defer(struct Parser* p, struct Parser_Error* error)
{
    struct Token defer_token = CHECK(parser_need(p, TOKEN_WORD_DEFER, error));

    struct Span span = defer_token.span;
    struct Statement_Value_Defer defer = { 0 };
    if (parser_probe(p, TOKEN_CURLY_OPEN)) {
        struct Block_Node block = CHECK(parser_block_node(p, error));
        span = span_merge(span, block.span);
        defer.block = block;
    } else {
        struct Expression* expression = CHECK(parser_expression(p, error));
        span = span_merge(span, expression->span);
        defer.expression = expression;
    }

    union Statement_Value value = { .defer = defer };
    return statement_new(STATEMENT_DEFER, value, span, defer_token.location);
}

struct Statement*
parser_statement(struct Parser* p, struct Parser_Error* error)
{
    struct Token token = parser_peek(p);

    // skip empty statements.
    while (token_ends_statement(&token)) {
        if (parser_reached_end(p)) return nil;
        parser_next(p);
        token = parser_peek(p);
    }

    if (token_can_begin_declaration(&token)) return parser_statement_declaration(p, error);

    switch (token.kind) {
    case TOKEN_WORD_IF:
        return parser_statement_conditional(p, error);
    case TOKEN_WORD_FOR:
        return parser_statement_for(p, error);
    case TOKEN_WORD_WHILE:
        return parser_statement_while(p, error);
    case TOKEN_CURLY_OPEN:
        return parser_statement_block(p, error);
    case TOKEN_WORD_RETURN:
        return parser_statement_return(p, error);
    case TOKEN_WORD_BREAK:
        return parser_statement_break(p, error);
    case TOKEN_WORD_CONTINUE:
        return parser_statement_continue(p, error);
    case TOKEN_WORD_DEFER:
        return parser_statement_defer(p, error);
    default:
        break;
    }

    struct Expression* expression = CHECK(parser_expression(p, error));

    // expand by one byte to include the statement terminator.
    struct Span span = span_expand(expression->span, 1);
    union Statement_Value value = { .expression.inner = expression };
    return statement_new(STATEMENT_EXPRESSION, value, span, expression->location);
}

// parse the lexer tokens into a single AST.
// note: it was either `parser_parse` or this. :)
struct Tree
parser_do_your_thing(struct Parser* p, struct Parser_Error* error)
{
    struct Statement* head = nil;

    struct Statement* current = nil;
    while (!parser_reached_end(p)) {
        struct Statement* next = CHECK_RETURN(parser_statement(p, error), struct Tree);
        if (!next) break; // on eof

        CHECK_RETURN(parser_end_statement(p, error), struct Tree);

        if (current) {
            current->next = next;
        } else {
            head = next;
        }
        current = next;
    }

    *error = parser_error_none();
    return (struct Tree){ head };
}

#undef CHECK
#undef CHECK_RETURN