1 files changed, 105 insertions, 0 deletions
diff --git a/src/commands/tac.c b/src/commands/tac.c
new file mode 100644
index 0000000..fbd0f74
--- /dev/null
+++ b/src/commands/tac.c
@@ -0,0 +1,105 @@
+#include "../command.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <limits.h>
+
+static void help(void) {
+    printf("Usage: tac [FILE]...\n\n");
+    printf("Concatenate FILEs and print them in reverse\n");
+}
+
+static void print_range(FILE* file, int start, int end) {
+    int len = end - start;
+    fseek(file, start, SEEK_SET);
+    for (int i = 0; i < len; i++) {
+        putchar(getc(file));
+    }
+    fflush(stdout);
+}
+
+static char stdin_path[PATH_MAX];
+
+static FILE* read_stdin() {
+    static bool read;
+    static FILE* file;
+
+    if (read) goto finished;
+    read = true;
+    
+    srand(time(NULL));
+    int r = rand() % 1000000;
+    sprintf(stdin_path, "/tmp/%d.tac", r);
+    file = get_file(stdin_path, "w");
+
+    char c;
+    while((c = getchar()) != EOF) putc(c, file);
+    fclose(file);
+
+    file = get_file(stdin_path, "r");
+
+finished:
+    return file;
+}
+
+static void parse_file(FILE* file, struct Stack* stack) {
+    char buf[1024];
+    int read;
+    int total = 1;
+
+    stack_push_int(stack, 0);
+    rewind(file);
+    while ((read = fread(buf, 1, 1024, file)) > 0) {
+        for (int i = 0; i < read; i++) {
+            char c = buf[i];
+            if (c != '\n') continue;
+            stack_push_int(stack, total + i);
+        }
+        total += read;
+    }
+}
+
+static void tac_file(FILE* file) {
+    struct Stack stack;
+    stack_init(&stack, 80);
+
+    parse_file(file, &stack);
+
+    rewind(file);
+    int last, current;
+    if (!stack_pop_int(&stack, &last)) goto cleanup;
+    while(stack_pop_int(&stack, &current)) {
+        print_range(file, current, last);
+        last = current;
+    }
+
+cleanup:
+    stack_free(&stack);
+}
+
+COMMAND(tac) {
+
+    parse_help(argc, argv, help);
+
+    FILE* in = read_stdin();
+
+    if (argc < 1) {
+        tac_file(in);
+        return EXIT_SUCCESS;
+    }
+
+    for (int i = 0; i < argc; i++) {
+        FILE* file = get_file(argv[i], "r");
+        if (file == stdin) {
+            tac_file(in);
+        } else {
+            tac_file(file);
+            fclose(file);
+        }
+    }
+
+    fclose(in);
+    remove(stdin_path);
+
+    return EXIT_SUCCESS;
+}