wire: add var_int parsing functions

so we can put and pull bitcoin 'var_int' length types from the
wire.

for more info on variable integers, see http://learnmeabitcoin.com/glossary/varint

wire/fromwire.c

@@ -101,6 +101,27 @@ bool fromwire_bool(const u8 **cursor, size_t *max)
 	return ret;
 }
 
+u64 fromwire_var_int(const u8 **cursor, size_t *max)
+{
+	if (*max < 1) {
+		fromwire_fail(cursor, max);
+		return 0;
+	}
+
+	u8 flag = fromwire_u8(cursor, max);
+
+	switch(flag) {
+	case 0xff:
+		return fromwire_u64(cursor, max);
+	case 0xfe:
+		return (u64)fromwire_u32(cursor, max);
+	case 0xfd:
+		return (u64)fromwire_u16(cursor, max);
+	default:
+		return (u64)flag;
+	}
+}
+
 void fromwire_pubkey(const u8 **cursor, size_t *max, struct pubkey *pubkey)
 {
 	u8 der[PUBKEY_DER_LEN];

wire/towire.c

@@ -54,6 +54,22 @@ void towire_bool(u8 **pptr, bool v)
 	towire(pptr, &val, sizeof(val));
 }
 
+void towire_var_int(u8 **pptr, const u64 val)
+{
+	if (val < 0xfd) {
+		towire_u8(pptr, (u8)val);
+	} else if (val <= 0xffff) {
+		towire_u8(pptr, 0xfd);
+		towire_u16(pptr, (u16)val);
+	} else if (val <= 0xffffffff) {
+		towire_u8(pptr, 0xfe);
+		towire_u32(pptr, (u32)val);
+	} else {
+		towire_u8(pptr, 0xff);
+		towire_u64(pptr, val);
+	}
+}
+
 void towire_pubkey(u8 **pptr, const struct pubkey *pubkey)
 {
 	u8 output[PUBKEY_DER_LEN];

wire/wire.h

@@ -67,6 +67,7 @@ void towire_u64(u8 **pptr, u64 v);
 void towire_double(u8 **pptr, const double *v);
 void towire_pad(u8 **pptr, size_t num);
 void towire_bool(u8 **pptr, bool v);
+void towire_var_int(u8 **pptr, const u64 val);
 
 void towire_u8_array(u8 **pptr, const u8 *arr, size_t num);
 
@@ -83,6 +84,7 @@ u32 fromwire_u32(const u8 **cursor, size_t *max);
 u64 fromwire_u64(const u8 **cursor, size_t *max);
 void fromwire_double(const u8 **cursor, size_t *max, double *v);
 bool fromwire_bool(const u8 **cursor, size_t *max);
+u64 fromwire_var_int(const u8 **cursor, size_t *max);
 void fromwire_secret(const u8 **cursor, size_t *max, struct secret *secret);
 void fromwire_privkey(const u8 **cursor, size_t *max, struct privkey *privkey);
 void fromwire_pubkey(const u8 **cursor, size_t *max, struct pubkey *pubkey);