Minimal templating in C

README.md

What is it?

Proof of concept.

We can create arbitrary templates like index.template.h and translate them with ctrans into valid C code that we can later call as shown in run.c

$ make
gcc ctrans.c -o ctrans
./ctrans index.template.h > index.h
gcc -I. run.c
./a.out
. . first line         >end
astart  end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
 end
\tsecond line    <<
John Doe

ctrans.c

// initially generated with https://aistudio.google.com/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <sys/stat.h>

char const* filename = NULL;

void print(char c) {
    if      (c == '\"') printf("\\\"");
    else if (c == '\r') printf("\\r");
    else if (c == '\n') printf("\\n");
    else if (c == '\\') printf("\\\\");
    else printf("%c", c);
}

void process(char const* text) {
    char const* cursor = text;
    char const* start_tag;
    
    while ((start_tag = strstr(cursor, "{{")) != NULL) {
        // Print text before tag
        if (start_tag != cursor) {
            printf("  sb_append(sb, \"");
            while (cursor < start_tag) {
                print(*cursor++);
            }
            printf("\");\n");
        }
        
        // Find end of tag
        const char* end_tag = strstr(start_tag, "}}");
        if (!end_tag) break;
        
        // Extract and print code
        printf("  ");
        const char* code_ptr = start_tag + 2;
        while (code_ptr < end_tag) {
            printf("%c", *code_ptr++);
        }
        
        cursor = end_tag + 2;
    }
    
    // Print remaining line content
    if (*cursor != '\0') {
        printf("  sb_append(sb, \"");
        while (*cursor) {
            print(*cursor++);
        }
        printf("\");\n");
    }
}

int main(int argc, char** argv) {
    FILE* in;
    struct stat st;
    char *buffer;

    if (argc == 2) {
        filename = argv[1];
        in       = fopen(filename, "r");
        if (!in) {
            perror("Can't open specified file");
            return EXIT_FAILURE;
        } 
        if (fstat(fileno(in), &st)) {
            perror("Can't get file stat");
            return EXIT_FAILURE;
        }
        buffer = malloc(st.st_size + 1);
        if (fread(buffer, 1, st.st_size, in) == 0) {
            perror("Can't read specified file");
            return EXIT_FAILURE;
        };
        buffer[st.st_size] = '\0';
    }

    printf("#include <string-builder.h>\n\n");
    printf("struct Context;\n\n");
    printf("void render_template(struct Context* ctx, struct StringBuilder* sb);\n\n");
    printf("#ifdef IMPLEMENTATION\n\n");
    printf("void render_template(struct Context* ctx, struct StringBuilder* sb) {\n");

    process(buffer);

    printf("}\n\n");
    printf("#endif  // IMPLEMENTATION\n");

    free(buffer);
    return EXIT_SUCCESS;
}

index.template.h

. . first line         >end
astart {{ for (int i = 0; i < 10; ++i) { }} end
\tsecond line    <<
{{ } sb_append(sb, ctx->user->name); }}

Makefile

run: a.out
	./a.out

ctrans: ctrans.c
	gcc ctrans.c -o ctrans

index.h: index.template.h ctrans
	./ctrans index.template.h > index.h

a.out: index.h
	gcc -I. run.c

run.c

#include <stdio.h>

struct User {
        char const* name;
};

struct Context {
        struct User *user;
};

#define IMPLEMENTATION
#define render_template index_page_template
#include "index.h"
#undef render_template

int main() {
        struct StringBuilder sb = {0};
        struct User user = {"John Doe"};
        struct Context ctx = {&user};
        index_page_template(&ctx, &sb);
        sb.data[sb.len] = '\0';
        puts(sb.data);
}

string-builder.h

#ifndef _STRING_BUILDER_H_
#define _STRING_BUILDER_H_

struct StringBuilder;

void sb_finalize(struct StringBuilder* sb);

#define PRINTF_LIKE __attribute__((format(printf, 2, 3)))
PRINTF_LIKE void sb_appendf(struct StringBuilder* sb, char const* fmt, ...);

#define sb_append(sb, text) sb_appendf((sb), "%s", (text))

#ifdef IMPLEMENTATION

#include <assert.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

struct StringBuilder {
        char data[1024 * 1024];
        size_t len;
};

void
sb_finalize(struct StringBuilder* sb)
{
        assert(sb->len < sizeof(sb->data) && "Too small internal buffer");
        sb->data[sb->len] = '\0';
}

void
sb_appendf(struct StringBuilder* sb, char const* fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        size_t free_space = sizeof(sb->data) - sb->len - 1;
        sb->len += vsnprintf(sb->data + sb->len, free_space, fmt, args);
        va_end(args);
}

#endif  // _STRING_BUILDER_H_

@oduortoni

The complex macro or the simple $ sign string replacement.

#define $ (*ctx) is not a complex macro. It is a macro that enforces specific
syntax. Difference between $user.name and $.user.name is barely noticeable,
yet later is completely valid C code that simplifies the logic of translation
tool.

Drawback of what exactly?

{{ for (int i = 0; i < $.user.friends.len; ++i) { }}
    {{ char *name = get_friend_name($.user.friends.ids[i]); }}
    <li>{{ "%s", name }}</li>
{{ } }}

In situation like this, you can't use String mode because it doens't start
with the $. so you have to use Format mode.

https://github.com/koutoftimer/zc