Task: Mad Libs
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define err(...) fprintf(stderr, ## __VA_ARGS__), exit(1)


typedef struct {
    char *data;
    size_t alloc;
    size_t length;
} dstr;

inline int dstr_space(dstr *s, size_t grow_amount)
{
    return s->length + grow_amount < s->alloc;
}

int dstr_grow(dstr *s)
{
    s->alloc *= 2;
    char *attempt = realloc(s->data, s->alloc);

    if (!attempt) return 0;
    else s->data = attempt;

    return 1;
}

dstr* dstr_init(const size_t to_allocate)
{
    dstr *s = malloc(sizeof(dstr));
    if (!s) goto failure;

    s->length = 0;
    s->alloc  = to_allocate;
    s->data   = malloc(s->alloc);

    if (!s->data) goto failure;

    return s;

failure:
    if (s->data) free(s->data);
    if (s) free(s);
    return NULL;
}

void dstr_delete(dstr *s)
{
    if (s->data) free(s->data);
    if (s) free(s);
}

dstr* readinput(FILE *fd)
{
    static const size_t buffer_size = 4096;
    char buffer[buffer_size];

    dstr *s = dstr_init(buffer_size);
    if (!s) goto failure;

    while (fgets(buffer, buffer_size, fd)) {
        while (!dstr_space(s, buffer_size))
            if (!dstr_grow(s)) goto failure;

        strncpy(s->data + s->length, buffer, buffer_size);
        s->length += strlen(buffer);
    }

    return s;

failure:
    dstr_delete(s);
    return NULL;
}

void dstr_replace_all(dstr *story, const char *replace, const char *insert)
{
    const size_t replace_l = strlen(replace);
    const size_t insert_l  = strlen(insert);
    char *start = story->data;

    while ((start = strstr(start, replace))) {
        if (!dstr_space(story, insert_l - replace_l))
            if (!dstr_grow(story)) err("Failed to allocate memory");

        if (insert_l != replace_l) {
            memmove(start + insert_l, start + replace_l, story->length -
                    (start + replace_l - story->data));

            
            story->length += insert_l - replace_l;
            story->data[story->length] = 0;
        }

        memmove(start, insert, insert_l);
    }
}

void madlibs(dstr *story)
{
    static const size_t buffer_size = 128;
    char insert[buffer_size];
    char replace[buffer_size];

    char *start,
         *end = story->data;

    while (start = strchr(end, '<')) {
        if (!(end = strchr(start, '>'))) err("Malformed brackets in input");

        
        strncpy(replace, start, end - start + 1);
        replace[end - start + 1] = '\0';

        printf("Enter value for field %s: ", replace);

        fgets(insert, buffer_size, stdin);
        const size_t il = strlen(insert) - 1;
        if (insert[il] == '\n')
            insert[il] = '\0';

        dstr_replace_all(story, replace, insert);
    }
    printf("\n");
}

int main(int argc, char *argv[])
{
    if (argc < 2) return 0;

    FILE *fd = fopen(argv[1], "r");
    if (!fd) err("Could not open file: '%s\n", argv[1]);

    dstr *story = readinput(fd); fclose(fd);
    if (!story) err("Failed to allocate memory");

    madlibs(story);
    printf("%s\n", story->data);
    dstr_delete(story);
    return 0;
}

Go: package main

import (
    "bufio"
    "fmt"
    "io/ioutil"
    "log"
    "os"
    "regexp"
    "strings"
)

func main() {
    pat := regexp.MustCompile("<.+?>")
    if len(os.Args) != 2 {
        fmt.Println("usage: madlib <story template file>")
        return
    }
    b, err := ioutil.ReadFile(os.Args[1])
    if err != nil {
        log.Fatal(err)
    }
    tmpl := string(b)
    s := []string{}          
    m := map[string]string{} 
    for _, p := range pat.FindAllString(tmpl, -1) {
        if _, ok := m[p]; !ok {
            m[p] = ""
            s = append(s, p)
        }
    }
    fmt.Println("Enter replacements:")
    br := bufio.NewReader(os.Stdin)
    for _, p := range s {
        for {
            fmt.Printf("%s: ", p[1:len(p)-1])
            r, isPre, err := br.ReadLine()
            if err != nil {
                log.Fatal(err)
            }
            if isPre {
                log.Fatal("you're not playing right. :P")
            }
            s := strings.TrimSpace(string(r))
            if s == "" {
                fmt.Println("  hmm?")
                continue
            }
            m[p] = s
            break
        }
    }
    fmt.Println("\nYour story:\n")
    fmt.Println(pat.ReplaceAllStringFunc(tmpl, func(p string) string {
        return m[p]
    }))
}