porting bash func for truncating home dir to C (mostly for practice)

digressions/.gitignore

@@ -1 +1,2 @@
 /temperatures
+/truncpwd

digressions/Makefile

@@ -1,5 +1,5 @@
 CFLAGS += -g -Wall -Wextra
 
-all: temperatures
+all: temperatures truncpwd
 
 .PHONY: all

digressions/truncpwd.c

@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <unistd.h>
+
+
+#define BUFSIZE 2048 + 1
+#define DEFAULT_TRUNCPWD_LEN "40"
+
+
+void substr(char *dest, char *src, int offset, int len) {
+    int i;
+    for (i = 0; i < len && src[offset + i] != '\0'; i++) {
+        dest[i] = src[offset + i];
+    }
+    dest[i] = '\0';
+}
+
+
+void with_home_as_tilde(char *dest, char *src, char *home) {
+    size_t src_len = strlen(src);
+    size_t home_len = strlen(home);
+
+    if (strncmp(src, home, home_len) == 0) {
+        char less_home[BUFSIZE];
+        substr(less_home, src, home_len, src_len - home_len);
+        sprintf(dest, "~%s", less_home);
+    } else {
+        sprintf(dest, "%s", src);
+    }
+}
+
+
+int main(void)
+{
+    char pwd[BUFSIZE];
+    const char *home = getenv("HOME");
+    const char *truncpwd_len_str = getenv("TRUNCPWD_LEN");
+    if (truncpwd_len_str == NULL) {
+        truncpwd_len_str = DEFAULT_TRUNCPWD_LEN;
+    }
+    unsigned int truncpwd_len = (unsigned int)atoi(truncpwd_len_str);
+
+    const char *truncpwd_leader = getenv("TRUNCPWD_LEADER");
+    if (truncpwd_leader == NULL) {
+        truncpwd_leader = "...";
+    }
+
+    if (getcwd(pwd, sizeof(pwd)) == NULL) {
+        fprintf(stderr, "truncpwd: %s\n", strerror(errno));
+        return 1;
+    }
+
+    size_t pwd_len = strlen(pwd);
+
+    if (strncmp(home, pwd, pwd_len) == 0) {
+        printf("~\n");
+    }
+
+    char tilde_subbed[BUFSIZE];
+    with_home_as_tilde(tilde_subbed, pwd, (char *)home);
+
+    size_t tilde_subbed_len = strlen(tilde_subbed);
+
+    if (tilde_subbed_len <= truncpwd_len) {
+        printf("%s\n", tilde_subbed);
+        return 0;
+    } else {
+        char last_n[truncpwd_len + 1];
+        int offset = tilde_subbed_len - truncpwd_len + 1;
+        substr(last_n, tilde_subbed, offset, truncpwd_len + 1);
+        printf("%s%s\n", truncpwd_leader, last_n);
+    }
+    return 0;
+}