write code to improve handling of paths, to be more consistent.

I don't think I want to place the local data paths there...

src/game/paths.c

@@ -0,0 +1,65 @@
+#include "paths.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+#include "../util/compat.h"
+
+char const* restrict path_dat = NULL;
+char const* restrict path_font = NULL;
+char const* restrict path_music = NULL;
+char const* restrict path_place_sfx = NULL;
+
+// gets the game's data path, returns 0 on failure, otherwise the datapath's string length
+static unsigned getdatpath(void) {
+    char const* home = getenv(unixonly("HOME") winonly("APPDATA")); // get the user data directory path (appropriated for each platform)
+    if (!home) home = ".";                                          // if this failed, set the path to `.`, which represents cwd
+
+    unsigned len = strlen(home);
+    len += 1 + 20 unixonly(+13); // add 21 bytes to the home length, to account for adding .local/share later
+    char* datpath = malloc(len);
+    if (!datpath) return 0;
+
+    // copy the data from home into the datapath
+    strcpy(datpath, home);
+
+#ifdef __unix__
+    // include the .local/share directory, if the HOME environment variable was valid
+    if (home[0] != '.') strcat(datpath, "/.local/share");
+    else {
+        // if the HOME directory wasn't defined, shrink the string
+        len -= 13;
+        void* ptr = realloc(datpath, len);
+        if (ptr) datpath = ptr; // likely doesn't actually change the pointer, but just to be sure
+    }
+#endif
+
+    strcat(datpath, PATH_SEP_STR "quinns_tetris_clone");
+    path_dat = datpath;
+    return len;
+}
+
+static inline char const* init_path(char const* const restrict str, unsigned len) {
+    void* ptr = malloc(len);
+    if (!ptr) return NULL;
+    strcpy(ptr, path_dat);
+    strcat(ptr, str);
+    return ptr;
+}
+
+int paths_init(void) {
+    unsigned len = getdatpath();
+    if (!len) return 1;
+
+    // these are explicitly static, as string literals just work like that
+    path_font = init_path("pixeldroid_botic-regular.ttf", len + 28);
+    path_music = init_path("korobeiniki.wav", len + 15);
+    path_place_sfx = init_path("place.wav", len + 9);
+    return -(!path_font || !path_music || !path_place_sfx);
+}
+
+void paths_free(void) {
+    free((void*)path_dat), path_dat = NULL;
+    free((void*)path_music), path_music = NULL;
+    free((void*)path_place_sfx), path_place_sfx = NULL;
+}