From 21cc24baa135c1a2b882ba67c1e0b2cf9e64edfc Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Sun, 21 Dec 2025 19:19:37 -0500
Subject: usr: sdk: Add vsnprintf() family helpers

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 usr/sdk/common/string/vsnprintf.c | 120 ++++++++++++++++++++++++++++++++++++++
 usr/sdk/inc/sdk/string.h          |  22 ++++++-
 2 files changed, 141 insertions(+), 1 deletion(-)
 create mode 100644 usr/sdk/common/string/vsnprintf.c

diff --git a/usr/sdk/common/string/vsnprintf.c b/usr/sdk/common/string/vsnprintf.c
new file mode 100644
index 0000000..8d78989
--- /dev/null
+++ b/usr/sdk/common/string/vsnprintf.c
@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2025, Ian Moffett.
+ * Provided under the BSD-3 clause.
+ */
+
+#include <sdk/types.h>
+#include <sdk/string.h>
+#include <sdk/stdarg.h>
+
+static inline void
+printc(char *buf, USIZE size, USIZE *off, char c)
+{
+    if (*off < size - 1) {
+        buf[(*off)++] = c;
+    }
+    buf[*off] = 0;
+}
+
+static void
+printstr(char *buf, USIZE size, USIZE *off, const char *s)
+{
+    while (*off < size - 1 && *s != '\0') {
+        buf[(*off)++] = *(s)++;
+    }
+    buf[*off] = 0;
+}
+
+int
+snprintf(char *s, USIZE size, const char *fmt, ...)
+{
+    va_list ap;
+    int ret;
+
+    va_start(ap, fmt);
+    ret = vsnprintf(s, size, fmt, ap);
+    va_end(ap);
+    return ret;
+}
+
+int
+vsnprintf(char *s, USIZE size, const char *fmt, va_list ap)
+{
+    USIZE tmp_len, num_len, off = 0;
+    USIZE num = 0;
+    char c, c1, num_buf[256] = {0};
+    const char *tmp_str;
+    BYTE pad_width = 0;
+
+    while (off < (size - 1)) {
+        while (*fmt && *fmt != '%') {
+            printc(s, size, &off, *fmt++);
+        }
+        if (*(fmt)++ == '\0' || off == size - 1) {
+            return off;
+        }
+
+        /*
+         * Handle a case where we have "%0N". For example:
+         * "%04d"
+         */
+        if (*fmt == '0') {
+            ++fmt;
+            while (*fmt >= '0' && *fmt <= '9') {
+                pad_width = pad_width * 10 + (*fmt - '0');
+                ++fmt;
+            }
+        }
+
+        c = *fmt++;
+        switch (c) {
+        case 'c':
+            c1 = (char )va_arg(ap, int);
+            printc(s, size, &off, c1);
+            break;
+        case '%':
+            printc(s, size, &off, c);
+            break;
+        case 'd':
+            num = va_arg(ap, QUAD);
+            itoa(num, num_buf, 10);
+
+            if (pad_width > 0) {
+                num_len = strlen(num_buf);
+                for (USIZE i = num_len; i < pad_width; ++i)
+                    printc(s, size, &off, '0');
+                pad_width = 0;
+            }
+            printstr(s, size, &off, num_buf);
+            break;
+        case 'p':
+            num = va_arg(ap, UQUAD);
+            itoa(num, num_buf, 16);
+            tmp_len = strlen(num_buf);
+
+            /* Add '0x' prefix */
+            printc(s, size, &off, '0');
+            printc(s, size, &off, 'x');
+
+            /* XXX: Assumes 64-bit */
+            for (USIZE i = 0; i < 18 - tmp_len; ++i) {
+                printc(s, size, &off, '0');
+            }
+
+            printstr(s, size, &off, num_buf + 2);
+            break;
+        case 'x':
+            num = va_arg(ap, UQUAD);
+            itoa(num, num_buf, 16);
+            tmp_len = strlen(num_buf);
+            printstr(s, size, &off, num_buf + 2);
+            break;
+        case 's':
+            tmp_str = va_arg(ap, const char *);
+            printstr(s, size, &off, tmp_str);
+            break;
+        }
+    }
+
+    return 0;
+}
diff --git a/usr/sdk/inc/sdk/string.h b/usr/sdk/inc/sdk/string.h
index 0f7ff27..965f66a 100644
--- a/usr/sdk/inc/sdk/string.h
+++ b/usr/sdk/inc/sdk/string.h
@@ -6,7 +6,8 @@
 #ifndef _SDK_STRING_H_
 #define _SDK_STRING_H_ 1
 
- #include <sdk/types.h>
+#include <sdk/types.h>
+#include <sdk/stdarg.h>
 
 /*
  * Returns the length of a string
@@ -39,4 +40,23 @@ void *memcpy(void *dest, const void *src, USIZE count);
  */
 char *itoa(QUAD value, char *buf, int base);
 
+/*
+ * Write a formatted string to a buffer
+ *
+ * @s: Target buffer
+ * @size: Maximum buffer size
+ * @fmt: Format string
+ * @<...>: Args
+ */
+int snprintf(char *s, USIZE size, const char *fmt, ...);
+
+/*
+ * Write a formatted string to a buffer using a va_list
+ *
+ * @s: Target buffer
+ * @size: Maximum buffer size
+ * @ap: Args pointer
+ */
+int vsnprintf(char *s, USIZE size, const char *fmt, va_list ap);
+
 #endif  /* !_SDK_STRING_H_ */
-- 
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