1 files changed, 90 insertions, 0 deletions
diff --git a/02_exercise/prompt_utils.c b/02_exercise/prompt_utils.c
new file mode 100644
index 0000000..168c088
--- /dev/null
+++ b/02_exercise/prompt_utils.c
@@ -0,0 +1,90 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include "array.h"
+#include "prompt_utils.h"
+
+char const *relative_path(char const *const from_dir, char const *const to_dir) {
+    // easiest cases first
+    {
+        if (strcmp(from_dir, to_dir) == 0) {
+            char *return_value = malloc(2);
+            strcpy(return_value, ".");
+            return return_value;
+        }
+    }
+    {
+        if (strcmp("/", from_dir) == 0) {
+            char *return_value = malloc(strlen(to_dir) + 2);
+            return_value[0] = '.';
+            memcpy(return_value + 1, to_dir, strlen(to_dir) + 1);
+            return return_value;
+        }
+    }
+
+    // splitting path into pieces so we can do strcmp on each of them
+    size_t const *const from_dir_indeces = get_separator_indices(from_dir, '/');
+    size_t const *const to_dir_indeces = get_separator_indices(to_dir, '/');
+
+    size_t from_dir_len = arrayLen(from_dir_indeces);
+    size_t to_dir_len = arrayLen(to_dir_indeces);
+
+    // finding the longest common substring
+    size_t array_len = from_dir_len < to_dir_len ? from_dir_len : to_dir_len;
+    size_t i = 0;
+    size_t common_position = 0;
+    for (; i < array_len - 1; ++i) {
+        if (from_dir_indeces[i + 1] != to_dir_indeces[i + 1]) {
+            break;
+        }
+        size_t index = from_dir_indeces[i];
+        size_t count = from_dir_indeces[i + 1] - from_dir_indeces[i];
+        if (strncmp(from_dir + index, to_dir + index, count) != 0) {
+            break;
+        }
+        common_position = from_dir_indeces[i + 1];
+    }
+
+    size_t levels_up = from_dir_len - i - 1;
+    char *return_value;
+    if (levels_up == 0) {
+        // equal dirs for whole length of path and not equal => subdir
+        size_t length = strlen(to_dir + common_position) + strlen("./") + 1;
+        return_value = malloc(length * sizeof(char));
+        strcpy(return_value, ".");
+        strcat(return_value, to_dir + common_position);
+    } else {
+        char const *const go_up = "/..";
+        size_t length = strlen(to_dir + common_position) + strlen("..") +
+                        strlen(go_up) * (levels_up - 1) + 1;
+        return_value = malloc(length * sizeof(char));
+        strcpy(return_value, "..");
+        for (size_t j = 0; j < levels_up - 1; ++j) {
+            strcat(return_value, go_up);
+        }
+        if (strcmp("/", to_dir) != 0) {
+            strcat(return_value, to_dir + common_position);
+        }
+    }
+
+    arrayRelease((void *)from_dir_indeces);
+    arrayRelease((void *)to_dir_indeces);
+
+    return return_value;
+}
+
+size_t *get_separator_indices(char const *const text, char seperator) {
+    size_t *indices;
+    arrayInit(indices);
+    char const *current = text;
+    if (strchr(current, seperator) == NULL) {
+        arrayRelease(indices);
+        return NULL;
+    }
+    while ((current = strchr(current, seperator)) != NULL) {
+        arrayPush(indices) = (size_t) (current - text);
+        ++current;
+    }
+    arrayPush(indices) = strlen(text);
+    return indices;
+}