devtools/fp16: conversion tool for fp16 format.

We use this in gossmap to represent htlc max/min approximations.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/fp16.c

@@ -0,0 +1,45 @@
+/* Simple tool to convert to/from our internal fp16 representation */
+#include "config.h"
+#include <ccan/err/err.h>
+#include <ccan/opt/opt.h>
+#include <common/fp16.h>
+#include <common/setup.h>
+#include <common/type_to_string.h>
+#include <inttypes.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+	fp16_t fp16val;
+	u64 u64min, u64max;
+	char *endp;
+
+	common_setup(argv[0]);
+
+	opt_register_noarg("-h|--help", opt_usage_and_exit,
+			   "[fp16:val|value]\n"
+			   "Converts to/from fp16, indicates ranges",
+			   "Get usage information");
+	opt_parse(&argc, argv, opt_log_stderr_exit);
+	if (argc != 2)
+		opt_usage_exit_fail("Expect 1 argument");
+
+	if (strstarts(argv[1], "fp16:"))
+		fp16val = strtoul(argv[1] + 5, &endp, 0);
+	else
+		fp16val = u64_to_fp16(strtoul(argv[1], &endp, 0), false);
+
+	if (*endp)
+		opt_usage_exit_fail("Expect valid number");
+
+	u64min = u64max = fp16_to_u64(fp16val);
+	while (u64_to_fp16(u64min-1, false) == fp16val)
+		u64min--;
+	while (u64_to_fp16(u64max+1, false) == fp16val)
+		u64max++;
+	printf("fp16:0x%x\n"
+	       "min %"PRIu64"\n"
+	       "max %"PRIu64"\n",
+	       fp16val, u64min, u64max);
+	common_shutdown();
+}