From dedc01ee1141c86c60abdff10a11e8b96514d441 Mon Sep 17 00:00:00 2001
From: Ashwin Paudel <62433766+Ashwin-Paudel@users.noreply.github.com>
Date: Sat, 31 Jul 2021 14:12:30 -0400
Subject: [PATCH] added printf function

---
 src/impl/kernel/main.c | 70 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/src/impl/kernel/main.c b/src/impl/kernel/main.c
index e1e1a3f..9b9b8dd 100644
--- a/src/impl/kernel/main.c
+++ b/src/impl/kernel/main.c
@@ -1,5 +1,75 @@
 #include "print.h"
 
+typedef __builtin_va_list va_list;
+#define va_start(ap, X) __builtin_va_start(ap, X)
+#define va_arg(ap, type) __builtin_va_arg(ap, type)
+#define va_end(ap) __builtin_va_end(ap)
+
+void _putc(unsigned char c) { print_char((char)c); }
+
+void printf(const char* fmt, ...)
+{
+    const char digits[] = "0123456789abcdef";
+    va_list ap;
+    char buf[10];
+    char* s;
+    unsigned char u;
+    int c, i, pad;
+
+    va_start(ap, fmt);
+    while ((c = *fmt++) != 0) {
+        if (c == '%') {
+            c = *fmt++;
+            if (c == 'l')
+                c = *fmt++;
+            switch (c) {
+            case 'c':
+                _putc(va_arg(ap, int));
+                continue;
+            case 's':
+                s = va_arg(ap, char*);
+                if (s == (void*)0)
+                    s = (char*)"<NULL>";
+                for (; *s; s++)
+                    _putc((int)*s);
+                continue;
+            case '%':
+                _putc('%');
+                continue;
+            case 'd':
+            case 'i':
+                c = 'u';
+            case 'u':
+            case 'x':
+            case 'X':
+                u = va_arg(ap, unsigned);
+                s = buf;
+                if (c == 'u') {
+                    do
+                        *s++ = digits[u % 10U];
+                    while (u /= 10U);
+                } else {
+                    pad = 0;
+                    for (i = 0; i < 8; i++) {
+                        if (pad)
+                            *s++ = '0';
+                        else {
+                            *s++ = digits[u % 16U];
+                            if ((u /= 16U) == false)
+                                pad = 1;
+                        }
+                    }
+                }
+                while (--s >= buf)
+                    _putc((int)*s);
+                continue;
+            }
+        }
+        _putc((int)c);
+    }
+    va_end(ap);
+}
+
 void kernel_main() {
     print_clear();
     print_set_color(PRINT_COLOR_YELLOW, PRINT_COLOR_BLACK);