/*
 * transpiler from a catskill intermediate translation unit,
 * to the c programming language, as the main translation target.
 *
 * walks a fully-lowered translation unit and emits (slightly ugly)
 * c source code.
 *
 * Copyright (c) 2025-2026, Mel G. <mel@rnrd.eu>
 *
 * SPDX-License-Identifier: MPL-2.0
 */

#pragma once

#include "catboot.h"

// the output of the transpilation pass.
// used for both file output and string buffer output.
struct Transpile_Output
{
    FILE* file;
    struct String_Buffer string;
};

struct Transpile_Output
transpile_output_from_file(FILE* file)
{
    check(file != nil, "transpile output file is nil");
    return (struct Transpile_Output){
        .file = file,
        .string = string_buffer_empty(),
    };
}

#define TRANSPILE_OUTPUT_MAX_LENGTH 262144 // 256 KiB

struct Transpile_Output
transpile_output_from_string(void)
{
    return (struct Transpile_Output){
        .file = nil,
        .string = string_buffer_new(TRANSPILE_OUTPUT_MAX_LENGTH),
    };
}

#define TRANSPILE_OUTPUT_MAX_WRITE_LENGTH 4096

void
transpile_output_write(struct Transpile_Output* output, const ascii* format, ...)
{
    va_list args;
    va_start(args, format);

    if (output->file) {
        vfprintf(output->file, format, args);
    } else {
        ascii buffer[TRANSPILE_OUTPUT_MAX_WRITE_LENGTH];
        int written = vsnprintf(buffer, TRANSPILE_OUTPUT_MAX_WRITE_LENGTH, format, args);
        check(written >= 0 && (uint)written < TRANSPILE_OUTPUT_MAX_LENGTH,
              "transpile output string buffer overflow");

        string_buffer_append_c_str(&output->string, buffer);
    }

    va_end(args);
}

struct String
transpile_output_string(struct Transpile_Output* output)
{
    check(output->file == nil, "transpile output is not a string");
    return string_buffer_to_string(&output->string);
}

FILE*
transpile_output_file(struct Transpile_Output* output)
{
    check(output->file != nil, "transpile output is not a file");
    return output->file;
}

struct Transpiler
{
    struct Transpile_Output output;
    struct Unit* unit;

    // current nesting depth, used to indent statement bodies.
    // mostly for decoration and easier observability!
    uint indent_level;

    // minimum precedence level of child expressions.
    // expressions use this to decide whether to wrap themselves in parentheses
    // in the case where their parent expression is stronger than them.
    uint expression_parent_precedence;
};

void
transpiler_new(struct Transpiler* transpiler, struct Transpile_Output output)
{
    *transpiler = (struct Transpiler){
        .output = output,
    };
}

#define TRANSPILE_PREAMBLE DATA_FOR_VISIT(struct Transpiler, transpiler);

#define TRANSPILE_WRITE(...) transpile_output_write(&transpiler->output, __VA_ARGS__)

// look up a type by id in the active translation unit.
// nil if the identifier is unknown.
struct Type*
transpile_type_at(struct Transpiler* transpiler, Type_Id id)
{
    if (id >= array_length(&transpiler->unit->types.entries)) return nil;
    return *array_at(struct Type*, &transpiler->unit->types.entries, id);
}

// the c-side form of a primitive catskill type.
// nil if the given id is not one of the primitives.
const ascii*
transpile_primitive_c_name(struct Transpiler* transpiler, Type_Id id)
{
    // TODO: far-out, eventually we want to wrap these types not only in an
    // empty "primitive" type shape, but actually define these concretely
    // as real types with functions in the catskill standard library!
    struct Type_Table* t = &transpiler->unit->types;
    if (id == t->primitive_int_id) return "integer";
    if (id == t->primitive_uint_id) return "uint";
    if (id == t->primitive_bool_id) return "bool";
    if (id == t->primitive_string_id) return "struct String";
    if (id == t->primitive_float_id) return "real";
    if (id == t->primitive_byte_id) return "byte";
    if (id == t->primitive_ascii_id) return "ascii";
    if (id == t->primitive_void_id) return "void";
    return nil;
}

// emit the c form of a use of a type.
void
transpile_visit_type_ref(struct Visit* visit, struct Type_Ref* ref)
{
    TRANSPILE_PREAMBLE

    struct Type* target = transpile_type_at(transpiler, ref->type_id);
    const ascii* primitive = transpile_primitive_c_name(transpiler, ref->type_id);

    if (primitive) {
        TRANSPILE_WRITE("%s", primitive);
    } else if (target && (target->kind == TYPE_STRUCTURE || target->kind == TYPE_VARIANT)) {
        TRANSPILE_WRITE("struct %s", string_c_str(target->name));
    } else if (target) {
        TRANSPILE_WRITE("%s", string_c_str(target->name));
    } else {
        TRANSPILE_WRITE("/* unknown type %lu */", ref->type_id);
    }

    // TODO: the other mods!
    FOR_EACH_ARRAY (enum Type_Modifier, mod, &ref->mods) {
        if (*mod == TYPE_MOD_REFERENCE) TRANSPILE_WRITE("*");
    }
}

void
transpile_visit_type_forward_structure(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("struct %s;\n", string_c_str(type->name));
}

void
transpile_visit_type_forward_variant(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("struct %s;\n", string_c_str(type->name));
}

void
transpile_visit_type_forward_function(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE

    // referenced function types are emitted as a typedef
    TRANSPILE_WRITE("typedef ");
    VISIT(visit_type_ref, &type->value.function.return_type);
    TRANSPILE_WRITE(" (*%s)(", string_c_str(type->name));
    if (array_length(&type->value.function.params) == 0) {
        TRANSPILE_WRITE("void");
    } else {
        bool first = true;
        FOR_EACH_ARRAY (struct Type_Ref, param, &type->value.function.params) {
            if (!first) TRANSPILE_WRITE(", ");
            VISIT(visit_type_ref, param);
            first = false;
        }
        if (type->value.function.variadic) TRANSPILE_WRITE(", ...");
    }
    TRANSPILE_WRITE(");\n");
}

void
transpile_visit_type_definition_alias(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE

    // aliases map to typedefs
    // TODO: maybe aliases are too much sugar for the lowered representation,
    // technically any reference to an alias could directly map to the underlying type.
    struct Type_Ref bare = type_ref_bare(type->value.alias.target_id);
    TRANSPILE_WRITE("typedef ");
    VISIT(visit_type_ref, &bare);
    TRANSPILE_WRITE(" %s;\n", string_c_str(type->name));
}

void
transpile_visit_type_definition_structure(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE

    TRANSPILE_WRITE("struct %s {\n", string_c_str(type->name));
    FOR_EACH_ARRAY (struct Field, field, &type->value.structure.fields) {
        TRANSPILE_WRITE("    ");
        VISIT(visit_type_ref, &field->type);
        TRANSPILE_WRITE(" %s;\n", string_c_str(field->name));
    }
    TRANSPILE_WRITE("};\n");
}

void
transpile_visit_type_definition_variant(struct Visit* visit, struct Type* type)
{
    TRANSPILE_PREAMBLE

    TRANSPILE_WRITE("struct %s {\n", string_c_str(type->name));
    TRANSPILE_WRITE("    uint32 tag;\n");
    bool any_payload = false;
    FOR_EACH_ARRAY (struct Variant_Case, one_case, &type->value.variant.cases) {
        if (one_case->has_payload) {
            any_payload = true;
            break;
        }
    }
    if (any_payload) {
        TRANSPILE_WRITE("    union {\n");
        FOR_EACH_ARRAY (struct Variant_Case, one_case, &type->value.variant.cases) {
            if (!one_case->has_payload) continue;
            TRANSPILE_WRITE("        ");
            VISIT(visit_type_ref, &one_case->payload);
            TRANSPILE_WRITE(" %s;\n", string_c_str(one_case->name));
        }
        TRANSPILE_WRITE("    } as;\n");
    }
    TRANSPILE_WRITE("};\n");
}

// transpilation pass 1 visit table
// emits forward declarations of types
struct Visit_Table transpile_forward_visit_functions = {
    .visit_type_ref = transpile_visit_type_ref,
    .visit_type_structure = transpile_visit_type_forward_structure,
    .visit_type_variant = transpile_visit_type_forward_variant,
    .visit_type_function = transpile_visit_type_forward_function,
};

// transpilation pass 2 visit table
// emits full definitions of types
struct Visit_Table transpile_definition_visit_functions = {
    .visit_type_ref = transpile_visit_type_ref,
    .visit_type_alias = transpile_visit_type_definition_alias,
    .visit_type_structure = transpile_visit_type_definition_structure,
    .visit_type_variant = transpile_visit_type_definition_variant,
    .visit_type_function = walk_type_primitive, // no-op, already done in pass 1
};

// the c-side name a function is emitted under.
// `main` becomes `catskill_main` so the runtime trailer can wrap it.
const ascii*
transpile_function_c_name(struct Function* function)
{
    if (function->is_main) return "catskill_main";
    return string_c_str(function->name);
}

// emits c-side format of a function header.
// shared by the signature and body passes.
void
transpile_function_emit_signature(struct Visit* visit, struct Function* function)
{
    TRANSPILE_PREAMBLE

    VISIT(visit_type_ref, &function->return_type);
    TRANSPILE_WRITE(" %s(", transpile_function_c_name(function));
    if (array_length(&function->params) == 0) {
        TRANSPILE_WRITE("void");
    } else {
        bool first = true;
        FOR_EACH_ARRAY (struct Param, param, &function->params) {
            if (!first) TRANSPILE_WRITE(", ");
            VISIT(visit_type_ref, &param->type);
            TRANSPILE_WRITE(" %s", string_c_str(param->name));
            first = false;
        }
        if (function->variadic) TRANSPILE_WRITE(", ...");
    }
    TRANSPILE_WRITE(")");
}

void
transpile_visit_function_signature(struct Visit* visit, struct Function* function)
{
    TRANSPILE_PREAMBLE
    transpile_function_emit_signature(visit, function);
    TRANSPILE_WRITE(";\n");
}

void
transpile_visit_function_body(struct Visit* visit, struct Function* function)
{
    TRANSPILE_PREAMBLE
    transpile_function_emit_signature(visit, function);
    TRANSPILE_WRITE(" ");
    if (function->body) {
        VISIT(visit_block, function->body);
    } else {
        TRANSPILE_WRITE("{\n}");
    }
    TRANSPILE_WRITE("\n");
}

void
transpile_emit_indent(struct Transpiler* transpiler)
{
    for (uint i = 0; i < transpiler->indent_level; ++i) TRANSPILE_WRITE("    ");
}

// dispatch on an expression with a parent-precedence value supplied by the caller.
void
transpile_emit_expression(
    struct Visit* visit, struct Expression* expression, uint parent_precedence)
{
    TRANSPILE_PREAMBLE
    transpiler->expression_parent_precedence = parent_precedence;
    VISIT(visit_expression, expression);
}

// precedence levels for non-binary operators! higher means tighter.
#define TRANSPILE_PRECEDENCE_UNARY 14
#define TRANSPILE_PRECEDENCE_POSTFIX 15
#define TRANSPILE_PRECEDENCE_PRIMARY 16

void
transpile_visit_expression_integer_literal(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("%ld", expression->value.integer_literal.value);
}

void
transpile_visit_expression_float_literal(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("%f", expression->value.float_literal.value);
}

void
transpile_visit_expression_string_literal(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    // TODO: emit a real, typed catskill string object once the standard
    // library has one and it can be wired in as a primitive.
    TRANSPILE_WRITE("\"%s\"", string_c_str(expression->value.string_literal.value));
}

void
transpile_visit_expression_boolean_literal(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("%s", expression->value.bool_literal.value ? "true" : "false");
}

void
transpile_visit_expression_name(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("%s", string_c_str(expression->value.name.name));
}

void
transpile_visit_expression_unary_operation(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE

    uint parent = transpiler->expression_parent_precedence;
    bool needs_parens = TRANSPILE_PRECEDENCE_UNARY < parent;
    if (needs_parens) TRANSPILE_WRITE("(");

    TRANSPILE_WRITE("%s", unary_operation_to_string(expression->value.unary_operator.operation));
    transpile_emit_expression(
        visit, expression->value.unary_operator.operand, TRANSPILE_PRECEDENCE_UNARY);

    if (needs_parens) TRANSPILE_WRITE(")");
}

void
transpile_visit_expression_binary_operation(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE

    enum Binary_Operation op = expression->value.binary_operator.operation;

    // c has no built-in power operator.
    // we route the operation via our own runtime library `pow_integer` function,
    // which performs fast exponentiation by squaring.
    if (op == BINARY_POWER) {
        TRANSPILE_WRITE("pow_integer(");
        transpile_emit_expression(visit, expression->value.binary_operator.left_operand, 0);
        TRANSPILE_WRITE(", ");
        transpile_emit_expression(visit, expression->value.binary_operator.right_operand, 0);
        TRANSPILE_WRITE(")");
        return;
    }

    uint my_precedence = binary_operation_precedence(op);
    uint parent = transpiler->expression_parent_precedence;
    bool needs_parens = my_precedence < parent;

    enum Binary_Operation_Associativity assoc = binary_operation_associativity(op);
    uint left_parent = (assoc == BINARY_ASSOCIATIVITY_RIGHT) ? my_precedence + 1 : my_precedence;
    uint right_parent = (assoc == BINARY_ASSOCIATIVITY_LEFT) ? my_precedence + 1 : my_precedence;

    if (needs_parens) TRANSPILE_WRITE("(");
    transpile_emit_expression(visit, expression->value.binary_operator.left_operand, left_parent);
    TRANSPILE_WRITE(" %s ", binary_operation_to_string(op));
    transpile_emit_expression(visit, expression->value.binary_operator.right_operand, right_parent);
    if (needs_parens) TRANSPILE_WRITE(")");
}

void
transpile_visit_expression_sizeof_operation(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("sizeof(");
    VISIT(visit_type_ref, &expression->value.sizeof_operator.target);
    TRANSPILE_WRITE(")");
}

void
transpile_visit_expression_call(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE

    transpile_emit_expression(visit, expression->value.call.subject, TRANSPILE_PRECEDENCE_POSTFIX);
    TRANSPILE_WRITE("(");
    Array(struct Call_Argument) args = expression->value.call.arguments;
    uint max_slot = 0;
    FOR_EACH_ARRAY (struct Call_Argument, a, &args) {
        if (a->slot > max_slot) max_slot = a->slot;
    }
    bool first = true;
    for (uint s = 0; s <= max_slot; ++s) {
        FOR_EACH_ARRAY (struct Call_Argument, a, &args) {
            if (a->slot != s) continue;
            if (!first) TRANSPILE_WRITE(", ");
            transpile_emit_expression(visit, a->value, 0);
            first = false;
        }
    }
    TRANSPILE_WRITE(")");
}

void
transpile_visit_expression_member(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    transpile_emit_expression(
        visit, expression->value.member.subject, TRANSPILE_PRECEDENCE_POSTFIX);
    // TODO: this only produces direct member references, however when the
    // subject is behind a pointer c forces us to use `x->y` to refer to them. fix!
    TRANSPILE_WRITE(".%s", string_c_str(expression->value.member.name));
}

void
transpile_visit_expression_subscript(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    transpile_emit_expression(
        visit, expression->value.subscript.subject, TRANSPILE_PRECEDENCE_POSTFIX);
    TRANSPILE_WRITE("[");
    transpile_emit_expression(visit, expression->value.subscript.index, 0);
    TRANSPILE_WRITE("]");
}

void
transpile_visit_expression_cast(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("(");
    VISIT(visit_type_ref, &expression->value.cast.target);
    TRANSPILE_WRITE(")");
    transpile_emit_expression(visit, expression->value.cast.operand, TRANSPILE_PRECEDENCE_UNARY);
}

void
transpile_visit_expression_construct(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE

    struct Type_Ref bare = type_ref_bare(expression->value.construct.type_id);
    TRANSPILE_WRITE("(");
    VISIT(visit_type_ref, &bare);
    TRANSPILE_WRITE("){");
    bool first = true;
    FOR_EACH_ARRAY (struct Construct_Field, field, &expression->value.construct.fields) {
        if (!first) TRANSPILE_WRITE(", ");
        if (field->name.length > 0) TRANSPILE_WRITE(".%s = ", string_c_str(field->name));
        transpile_emit_expression(visit, field->value, 0);
        first = false;
    }
    TRANSPILE_WRITE("}");
}

void
transpile_visit_expression_increment_decrement(struct Visit* visit, struct Expression* expression)
{
    TRANSPILE_PREAMBLE

    const ascii* op =
        expression->value.increment_decrement.operation == INCREMENT_DECREMENT_INCREMENT
            ? "++"
            : "--";
    if (expression->value.increment_decrement.prefix) {
        TRANSPILE_WRITE("%s", op);
        transpile_emit_expression(
            visit, expression->value.increment_decrement.subject, TRANSPILE_PRECEDENCE_UNARY);
    } else {
        transpile_emit_expression(
            visit, expression->value.increment_decrement.subject, TRANSPILE_PRECEDENCE_POSTFIX);
        TRANSPILE_WRITE("%s", op);
    }
}

// statements always emit their own indentation and trailing newline.
// the dispatcher writes the indentation then routes via normal statement apparatus.
void
transpile_visit_statement(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    transpile_emit_indent(transpiler);
    walk_statement(visit, stmt);
}

void
transpile_visit_statement_declaration(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    VISIT(visit_type_ref, &stmt->value.declaration.type);
    TRANSPILE_WRITE(" %s", string_c_str(stmt->value.declaration.name));
    if (stmt->value.declaration.initializer) {
        TRANSPILE_WRITE(" = ");
        transpile_emit_expression(visit, stmt->value.declaration.initializer, 0);
    }
    TRANSPILE_WRITE(";\n");
}

void
transpile_visit_statement_assign(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    transpile_emit_expression(visit, stmt->value.assign.lhs, 0);
    TRANSPILE_WRITE(" = ");
    transpile_emit_expression(visit, stmt->value.assign.rhs, 0);
    TRANSPILE_WRITE(";\n");
}

void
transpile_visit_statement_expression(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    transpile_emit_expression(visit, stmt->value.expression.inner, 0);
    TRANSPILE_WRITE(";\n");
}

void
transpile_visit_statement_conditional(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    bool first = true;
    FOR_EACH_ARRAY (struct If_Branch, branch, &stmt->value.conditional.branches) {
        if (first) {
            TRANSPILE_WRITE("if (");
            transpile_emit_expression(visit, branch->condition, 0);
            TRANSPILE_WRITE(") ");
        } else if (branch->condition) {
            TRANSPILE_WRITE(" else if (");
            transpile_emit_expression(visit, branch->condition, 0);
            TRANSPILE_WRITE(") ");
        } else {
            TRANSPILE_WRITE(" else ");
        }
        VISIT(visit_block, branch->body);
        first = false;
    }
    TRANSPILE_WRITE("\n");
}

void
transpile_visit_statement_loop(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("while (");
    transpile_emit_expression(visit, stmt->value.loop.condition, 0);
    TRANSPILE_WRITE(") ");
    VISIT(visit_block, stmt->value.loop.body);
    TRANSPILE_WRITE("\n");
}

void
transpile_visit_statement_return(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    if (stmt->value.return_value.value) {
        TRANSPILE_WRITE("return ");
        transpile_emit_expression(visit, stmt->value.return_value.value, 0);
        TRANSPILE_WRITE(";\n");
    } else {
        TRANSPILE_WRITE("return;\n");
    }
}

void
transpile_visit_statement_break(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE(void) stmt;
    TRANSPILE_WRITE("break;\n");
}

void
transpile_visit_statement_continue(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE(void) stmt;
    TRANSPILE_WRITE("continue;\n");
}

void
transpile_visit_statement_block(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    VISIT(visit_block, stmt->value.block.inner);
    TRANSPILE_WRITE("\n");
}

void
transpile_visit_statement_label(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("%s:;\n", string_c_str(stmt->value.label.name));
}

void
transpile_visit_statement_goto(struct Visit* visit, struct Statement* stmt)
{
    TRANSPILE_PREAMBLE
    TRANSPILE_WRITE("goto %s;\n", string_c_str(stmt->value.goto_target.target));
}

void
transpile_visit_block(struct Visit* visit, struct Block* block)
{
    TRANSPILE_PREAMBLE

    TRANSPILE_WRITE("{\n");
    transpiler->indent_level++;
    FOR_EACH_ARRAY (struct Statement*, statement, &block->statements) {
        VISIT(visit_statement, *statement);
    }
    transpiler->indent_level--;
    transpile_emit_indent(transpiler);
    TRANSPILE_WRITE("}");
}

// transpilation pass 3 visit table
// emits forward declarations of functions
struct Visit_Table transpile_signature_visit_functions = {
    .visit_type_ref = transpile_visit_type_ref,
    .visit_function = transpile_visit_function_signature,
};

// transpilation pass 4 visit table
// emits full definitions of functions
struct Visit_Table transpile_body_visit_functions = {
    .visit_type_ref = transpile_visit_type_ref,
    .visit_function = transpile_visit_function_body,
    .visit_block = transpile_visit_block,

    .visit_statement = transpile_visit_statement,
    .visit_statement_declaration = transpile_visit_statement_declaration,
    .visit_statement_assign = transpile_visit_statement_assign,
    .visit_statement_expression = transpile_visit_statement_expression,
    .visit_statement_conditional = transpile_visit_statement_conditional,
    .visit_statement_loop = transpile_visit_statement_loop,
    .visit_statement_return = transpile_visit_statement_return,
    .visit_statement_break = transpile_visit_statement_break,
    .visit_statement_continue = transpile_visit_statement_continue,
    .visit_statement_block = transpile_visit_statement_block,
    .visit_statement_label = transpile_visit_statement_label,
    .visit_statement_goto = transpile_visit_statement_goto,

    .visit_expression_integer_literal = transpile_visit_expression_integer_literal,
    .visit_expression_float_literal = transpile_visit_expression_float_literal,
    .visit_expression_string_literal = transpile_visit_expression_string_literal,
    .visit_expression_boolean_literal = transpile_visit_expression_boolean_literal,
    .visit_expression_name = transpile_visit_expression_name,
    .visit_expression_unary_operation = transpile_visit_expression_unary_operation,
    .visit_expression_binary_operation = transpile_visit_expression_binary_operation,
    .visit_expression_sizeof_operation = transpile_visit_expression_sizeof_operation,
    .visit_expression_call = transpile_visit_expression_call,
    .visit_expression_member = transpile_visit_expression_member,
    .visit_expression_subscript = transpile_visit_expression_subscript,
    .visit_expression_cast = transpile_visit_expression_cast,
    .visit_expression_construct = transpile_visit_expression_construct,
    .visit_expression_increment_decrement = transpile_visit_expression_increment_decrement,
};

// walk a lowered translation unit and emit c source into the transpiler's output.
// the order of the translation unit, and thus the final source output is fixed.
// preamble, then imports, type forwards, type definitions in topological dependency
// order, function forwads, function definitions, and runtime trailer (if main present).
int
transpile_unit(struct Transpiler* transpiler, struct Unit* unit)
{
    transpiler->unit = unit;

    visit_table_fill_defaults(&transpile_forward_visit_functions);
    visit_table_fill_defaults(&transpile_definition_visit_functions);
    visit_table_fill_defaults(&transpile_signature_visit_functions);
    visit_table_fill_defaults(&transpile_body_visit_functions);

    struct Visit visit_value = { .user_data = transpiler };
    struct Visit* visit = &visit_value;

    TRANSPILE_WRITE("#include \"core.c\"\n");

    FOR_EACH_ARRAY (struct Import, import, &unit->imports) {
        TRANSPILE_WRITE("#include \"%.*s\"\n", (int)import->path.length, import->path.data);
    }

    // pass 1: type forwards
    visit->table = &transpile_forward_visit_functions;
    FOR_EACH_ARRAY (struct Type*, type_slot, &unit->types.entries) {
        VISIT(visit_type, *type_slot);
    }

    // pass 2: type definitions
    visit->table = &transpile_definition_visit_functions;
    FOR_EACH_ARRAY (Type_Id, ordered_id, &unit->type_emission_order) {
        struct Type* type = *array_at(struct Type*, &unit->types.entries, *ordered_id);
        VISIT(visit_type, type);
    }

    // pass 3: function signatures
    visit->table = &transpile_signature_visit_functions;
    FOR_EACH_ARRAY (struct Function*, function_slot, &unit->functions.entries) {
        VISIT(visit_function, *function_slot);
    }

    // pass 4: function definitions
    visit->table = &transpile_body_visit_functions;
    FOR_EACH_ARRAY (struct Function*, function_slot, &unit->functions.entries) {
        VISIT(visit_function, *function_slot);
    }

    bool main_takes_args = false;
    bool has_main = false;
    FOR_EACH_ARRAY (struct Function*, function_slot, &unit->functions.entries) {
        struct Function* function = *function_slot;
        if (function->is_main) {
            has_main = true;
            main_takes_args = function->main_takes_args;
        }
    }

    if (has_main) {
        if (main_takes_args) TRANSPILE_WRITE("#define CATSKILL_MAIN_TAKES_ARGS\n");
        TRANSPILE_WRITE("#include \"runtime.c\"\n");
    }

    return 0;
}