diff --git a/functions.c b/functions.c
index 9daeafb..3e895bc 100644
--- a/functions.c
+++ b/functions.c
@@ -1 +1,164 @@
 test
+#include <assert.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include "str.h"
+#include "util.h"
+
+#define INIT_SZ 20
+
+void _str_ensure(str *s, int newsz) {
+    int tmp;
+    newsz += 1;
+    if (newsz < s->capacity) return;
+
+    tmp = s->capacity * 2;
+    if (tmp > newsz) newsz = tmp;
+    s->ptr = xrealloc(s->ptr, newsz);
+    s->capacity = newsz;
+}
+
+void str_init(str *s) {
+    assert(s);
+    s->len = 0;
+    s->capacity = INIT_SZ;
+    s->ptr = xmalloc(s->capacity);
+    s->ptr[0] = '\0';
+    printf("str_init: len=%d, capacity=%d\n", s->len, s->capacity);
+}
+
+void str_deinit(str *s) {
+    if (s->ptr) free(s->ptr);
+    s->ptr = NULL;
+    s->len = 0;
+    s->capacity = 0;
+    printf("str_deinit: memory freed\n");
+}
+
+void str_init_data(str *s, const char *initial) {
+    assert(s && initial);
+    int len = strlen(initial);
+    s->len = len;
+    s->capacity = len + 1;
+    if (s->capacity < INIT_SZ) s->capacity = INIT_SZ;
+    s->ptr = xmalloc(s->capacity);
+    strcpy(s->ptr, initial);
+    printf("str_init_data: '%s' (%d)\n", s->ptr, s->len);
+}
+
+char * str_data(str *s) {
+    assert(s);
+    return s->ptr;
+}
+
+void str_set(str *s, char *cstr) {
+    assert(s && cstr);
+    int len = strlen(cstr);
+    _str_ensure(s, len);
+    strcpy(s->ptr, cstr);
+    s->len = len;
+    printf("str_set: '%s' (%d)\n", s->ptr, s->len);
+}
+
+str str_copy(str *s) {
+    assert(s);
+    str new_str;
+    str_init(&new_str);
+    _str_ensure(&new_str, s->len);
+    strcpy(new_str.ptr, s->ptr);
+    new_str.len = s->len;
+    printf("str_copy: '%s' -> '%s'\n", s->ptr, new_str.ptr);
+    return new_str;
+}
+
+int str_count(str *s, char ch) {
+    assert(s);
+    int count = 0;
+    for (int i = 0; i < s->len; i++) {
+        if (s->ptr[i] == ch) count++;
+    }
+    printf("str_count: '%c' = %d in '%s'\n", ch, count, s->ptr);
+    return count;
+}
+
+void str_append(str *s, char *cstr) {
+    assert(s && cstr);
+    str_append_n(s, cstr, strlen(cstr));
+}
+
+void str_append_n(str *s, char *cstr, int len) {
+    assert(s && cstr);
+    _str_ensure(s, s->len + len);
+    memcpy(s->ptr + s->len, cstr, len);
+    s->len += len;
+    s->ptr[s->len] = '\0';
+    printf("str_append_n: '%s' (%d)\n", s->ptr, s->len);
+}
+
+void str_shrink(str *s, int len) {
+    assert(s && len >= 0);
+    if (len > s->len) len = s->len;
+    s->len = len;
+    s->ptr[s->len] = '\0';
+    printf("str_shrink: '%s' (%d)\n", s->ptr, s->len);
+}
+
+int str_find(str *s, char *substr) {
+    assert(s && substr);
+    char *p = strstr(s->ptr, substr);
+    int result = (p == NULL) ? -1 : p - s->ptr;
+    printf("str_find: '%s' in '%s' = %d\n", substr, s->ptr, result);
+    return result;
+}
+
+str str_sub(str *s, int start_idx, int length) {
+    assert(s && start_idx >= 0 && length >= 0);
+
+    str s2;
+    str_init(&s2);
+    
+    if (start_idx >= s->len) return s2;
+    
+    if (start_idx + length > s->len) length = s->len - start_idx;
+    
+    _str_ensure(&s2, length);
+    memcpy(s2.ptr, s->ptr + start_idx, length);
+    s2.ptr[length] = '\0';
+    s2.len = length;
+    
+    printf("str_sub: '%s'[%d:%d] = '%s'\n", s->ptr, start_idx, start_idx+length, s2.ptr);
+    return s2;
+}
+
+int str_replace(str *s, char *substr, char *replacement) {
+    assert(s && substr && replacement);
+    
+    int count = 0;
+    str result;
+    str_init(&result);
+    
+    int substr_len = strlen(substr);
+    int replacement_len = strlen(replacement);
+    
+    char *current = s->ptr;
+    char *found;
+    
+    while ((found = strstr(current, substr)) != NULL) {
+        int prefix_len = found - current;
+        if (prefix_len > 0) str_append_n(&result, current, prefix_len);
+        str_append_n(&result, replacement, replacement_len);
+        current = found + substr_len;
+        count++;
+    }
+    
+    if (*current != '\0') str_append(&result, current);
+    
+    free(s->ptr);
+    s->ptr = result.ptr;
+    s->len = result.len;
+    s->capacity = result.capacity;
+    
+    printf("str_replace: '%s' (%d)\n", s->ptr, count);
+    return count;
+}
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