Basic template engine

template.c

// Basic template engine
// $ cc -o template template.c
// $ printf '<title>{{title}}</title>\nHello, <b>{{name}}</b>!\n' >index.html
// $ ./template <index.html title='Example Page' name=skeeto
// Ref: https://old.reddit.com/r/C_Programming/comments/1pypl5t
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
    char key[256] = {};
    int  len      = 0;
    int  depth    = 0;
    int  dir      = 0;

    for (int c; (c = getchar()) != EOF;) {
        switch (c) {
        default:
            switch (depth) {
            case 2:
                if (len < (int)sizeof(key)) {
                    key[len++] = (char)c;
                }
                break;
            case 1:
                if (dir < 0) {
                    printf("{{%.*s}", len, key);
                } else {
                    putchar('{');
                }
                // fallthrough
            default:
                putchar(c);
                depth = 0;
            }
            break;

        case '{':
            switch (depth) {
            case 0:
            case 1:
                depth++;
                len = 0;
                break;
            case 2:
                printf("{{%.*s", len, key);
                len = 0;
                break;
            }
            dir = +1;
            break;

        case '}':
            switch (depth) {
            case 1:
                if (dir != -1) {
                    fputs("{}", stdout);
                } else {
                    for (int i = 1; i < argc; i++) {
                        char *arg = argv[i];
                        char *end = strchr(arg, '=');
                        if (end &&
                            end-arg==len &&
                            !memcmp(key, arg, (size_t)len)) {
                            fputs(end+1, stdout);
                        }
                    }
                }
                len = depth = 0;
                break;
            case 2:
                depth--;
                break;
            default:
                putchar('}');
            }
            dir = -1;
            break;
        }
    }
}