[libc] Add basic sprintf function

lib/libc/CMakeLists.txt

@@ -1,6 +1,7 @@
 
 add_library(c STATIC
   src/malloc.cpp
+  src/stdio.cpp
   )
 
 target_include_directories(c

lib/libc/include/stdio.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <stdarg.h>
+
+// int fprintf(FILE *stream, const char *format, ...);
+int printf(const char *format, ...);
+int sprintf(char *str, const char *format, ...);
+// int vfprintf(FILE *stream, const char *format, va_list arg);
+int vprintf(const char *format, va_list arg);
+int vsprintf(char *str, const char *format, va_list arg);

lib/libc/src/stdio.cpp

@@ -0,0 +1,90 @@
+#include "stdio.h"
+
+#include <stdint.h>
+
+namespace {
+
+uint32_t num_chars(uint32_t num, uint8_t base) {
+  uint32_t width = 0;
+  while (num > 0) {
+    num /= base;
+    width++;
+  }
+  return width;
+}
+
+int sprint_base(char *str, uint32_t num, uint32_t base) {
+  uint32_t width = num_chars(num, base);
+  if (width == 0) {
+    *str = '0';
+    return 1;
+  }
+
+  uint64_t place = 1;
+  while (num > 0) {
+    // FIXME: We seem to have an issue with loading globals.
+    const char *kHexChars = "0123456789abcdef";
+    str[width - place] = kHexChars[num % base];
+    place++;
+    num /= base;
+  }
+
+  return width;
+}
+
+}  // namespace
+
+int sprintf(char *str, const char *format, ...) {
+  va_list arg;
+  va_start(arg, format);
+  int ret = vsprintf(str, format, arg);
+  va_end(arg);
+  return ret;
+}
+
+int vsprintf(char *str, const char *format, va_list arg) {
+  uint32_t chars = 0;
+  while (*format != '\0') {
+    if (*format != '%') {
+      *str = *format;
+      chars++;
+      str++;
+      format++;
+      continue;
+    }
+    format++;
+    switch (*format) {
+      case '%':
+        *(str++) = *(format++);
+        chars++;
+        break;
+      case 'x': {
+        int width = sprint_base(str, va_arg(arg, uint32_t), 16);
+        if (width == -1) {
+          return -1;
+        }
+        chars += width;
+        str += width;
+        format++;
+        break;
+      }
+      case 'u': {
+        int width = sprint_base(str, va_arg(arg, uint32_t), 10);
+        if (width == -1) {
+          return -1;
+        }
+        chars += width;
+        str += width;
+        format++;
+        break;
+      }
+      default:
+        *(str++) = *(format++);
+        chars++;
+    }
+  }
+  *str = '\0';
+  chars++;
+
+  return chars;
+}