commit 12f8b73077b8b1587cc91e7dc23ee147d4177bd0
Author: Juhani Krekelä <juhani@krekelä.fi>
Date:   Sun Nov 20 06:52:54 2022 +0200

    First commit

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ae0b819
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.o
+baseconv_example
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..9608fdd
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,16 @@
+BIN = baseconv_example
+OBJS = baseconv.o main.o
+
+all: $(BIN)
+
+$(BIN): $(OBJS)
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(OBJS)
+
+main.o: baseconv.h
+
+clean:
+	rm -f $(BIN) $(OBJS)
+
+distclean: clean
+
+.PHONY: all clean distclean
diff --git a/baseconv.c b/baseconv.c
new file mode 100644
index 0000000..33ecad4
--- /dev/null
+++ b/baseconv.c
@@ -0,0 +1,47 @@
+#include <assert.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+static const char digits[] = "0123456789abcdefghijklmnopqrstuvwxyz";
+
+bool baseconv(char outbuf[], size_t bufsize, unsigned base, uintmax_t num) {
+	// Supported bases are 2 to 36 inclusive (though it could be higher
+	// if our digits string was longer
+	if (base < 2 || base > 36)
+		return false;
+
+	// Longest possible converted string is base-2, where we produce one
+	// character per bit of input, so if we size the buffer to fit it we
+	// can fit any converted string
+	char buf[sizeof(uintmax_t) * CHAR_BIT];
+
+	// Place the digits in the buffer in a little-endian order
+	size_t len = 0;
+	while (num) {
+		char digit = digits[num % base];
+		buf[len++] = digit;
+		num /= base;
+	}
+
+	// Special case: if the original input was zero, previous loop ran
+	// zero times, and there is nothing in the buffer. Place a single
+	// zero there.
+	if (!len)
+		buf[len++] = digits[0];
+
+	// Can the output buffer hold the converted string as well as the
+	// null terminator?
+	if (!bufsize || bufsize - 1 < len)
+		return false;
+
+	// Copy the digits to the output buffer, reversing the order
+	for (size_t i = 0; i < len; i++)
+		outbuf[i] = buf[len - 1 - i];
+
+	// Null-terminate
+	outbuf[len] = '\0';
+
+	return true;
+}
diff --git a/baseconv.h b/baseconv.h
new file mode 100644
index 0000000..72f5df1
--- /dev/null
+++ b/baseconv.h
@@ -0,0 +1 @@
+bool baseconv(char outbuf[], size_t bufsize, unsigned base, uintmax_t num);
diff --git a/main.c b/main.c
new file mode 100644
index 0000000..826fbc1
--- /dev/null
+++ b/main.c
@@ -0,0 +1,47 @@
+#include <limits.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "baseconv.h"
+
+int main(void) {
+	uintmax_t num;
+	printf("Number? ");
+	if (scanf("%ju", &num) != 1) {
+		fprintf(stderr, "Invalid input\n");
+		return 1;
+	}
+
+	unsigned base;
+	printf("Base? ");
+	if (scanf("%u", &base) != 1) {
+		fprintf(stderr, "Invalid input\n");
+		return 1;
+	}
+
+	size_t bufsize;
+	printf("Buffer size? ");
+	if (scanf("%zu", &bufsize) != 1) {
+		fprintf(stderr, "Invalid input\n");
+		return 1;
+	}
+
+	char *buf = malloc(bufsize);
+	if (!buf) {
+		fprintf(stderr, "Failed to allocate %zu bytes\n", bufsize);
+		return 1;
+	}
+
+	if (!baseconv(buf, bufsize, base, num)) {
+		fprintf(stderr, "Conversion error\n");
+		free(buf);
+		return 1;
+	}
+
+	printf("%s\n", buf);
+
+	free(buf);
+}