Make it rebuild only when something changes

nob.c

@@ -38,6 +38,13 @@ bool render_clangd_config(void) {
 }
 
 bool build_assembly(Nob_Cmd *cmd) {
+    const char *output = BUILD_FOLDER "boot.o";
+    const char *input = SRC_FOLDER "boot.s";
+    int rebuild = nob_needs_rebuild1(output, input);
+
+    if (!rebuild)
+        return true;
+
     nob_cmd_append(cmd, "i686-elf-as", "-o", BUILD_FOLDER "boot.o", SRC_FOLDER "boot.s");
     if (!nob_cmd_run(cmd))
         return false;
@@ -46,6 +53,13 @@ bool build_assembly(Nob_Cmd *cmd) {
 }
 
 bool build_c(Nob_Cmd *cmd) {
+    const char *output = BUILD_FOLDER "kernel.o";
+    const char *input = SRC_FOLDER "kernel.c";
+    int rebuild = nob_needs_rebuild1(output, input);
+
+    if (!rebuild)
+        return true;
+
     nob_cmd_append(cmd,
                    "i686-elf-gcc",
                    "-std=gnu99",
@@ -62,6 +76,13 @@ bool build_c(Nob_Cmd *cmd) {
 }
 
 bool link_everything(Nob_Cmd *cmd) {
+    const char *output = BUILD_FOLDER "kernel";
+    const char *input[] = {SRC_FOLDER "linker.ld", BUILD_FOLDER "kernel.o", BUILD_FOLDER "boot.o"};
+    int rebuild = nob_needs_rebuild(output, input, NOB_ARRAY_LEN(input));
+
+    if (!rebuild)
+        return true;
+
     Nob_File_Paths objectFiles = {0};
     nob_da_append(&objectFiles, BUILD_FOLDER "kernel.o");
     nob_da_append(&objectFiles, BUILD_FOLDER "boot.o");
@@ -73,6 +94,7 @@ bool link_everything(Nob_Cmd *cmd) {
                    "-O2",
                    "-nostdlib");
 
+    // Add objectFiles as input
     for (size_t i = 0; i < objectFiles.count; ++i) {
         nob_cmd_append(cmd, objectFiles.items[i]);
     }
@@ -116,6 +138,13 @@ bool clean_dir(const char *dir_path) {
 }
 
 bool build_bootable_image(Nob_Cmd *cmd) {
+    const char *output = BUILD_FOLDER "kernel.iso";
+    const char *input[] = {BUILD_FOLDER "kernel", SRC_FOLDER "grub.cfg"};
+    int rebuild = nob_needs_rebuild(output, input, NOB_ARRAY_LEN(input));
+
+    if (!rebuild)
+        return true;
+
     if (!nob_mkdir_if_not_exists(BUILD_FOLDER "isodir"))
         return false;
     if (!nob_mkdir_if_not_exists(BUILD_FOLDER "isodir/boot"))

src/kernel.c

@@ -101,5 +101,5 @@ void kernel_main(void) {
     terminal_initialize();
 
     /* Newline support is left as an exercise. */
-    terminal_writestring("Hello, kernel World!\n");
+    terminal_writestring("Hello world from kernel.\n");
 }