From: Alexis Laferrière <alexis.laf@xymus.net>
Date: Tue, 8 Aug 2017 16:23:18 +0000 (-0400)
Subject: core: support signed ints and custom capacity in `Int::to_bytes`
X-Git-Url: http://nitlanguage.org

core: support signed ints and custom capacity in `Int::to_bytes`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
---

diff --git a/lib/core/bytes.nit b/lib/core/bytes.nit
index e375e60..6ef4a90 100644
--- a/lib/core/bytes.nit
+++ b/lib/core/bytes.nit
@@ -651,7 +651,7 @@ private class BytesIterator
 end
 
 redef class Int
-	# A big-endian representation of self.
+	# A signed big-endian representation of `self`
 	#
 	# ~~~
 	# assert     1.to_bytes.hexdigest ==     "01"
@@ -661,43 +661,86 @@ redef class Int
 	# assert 65536.to_bytes.hexdigest == "010000"
 	# ~~~
 	#
+	# Negative values are converted to their two's complement.
+	# Be careful as the result can be ambiguous.
+	#
+	# ~~~
+	# assert     (-1).to_bytes.hexdigest ==     "FF"
+	# assert    (-32).to_bytes.hexdigest ==     "E0"
+	# assert   (-512).to_bytes.hexdigest ==   "FE00"
+	# assert (-65794).to_bytes.hexdigest == "FEFEFE"
+	# ~~~
+	#
+	# Optionally, set `n_bytes` to the desired number of bytes in the output.
+	# This setting can disambiguate the result between positive and negative
+	# integers. Be careful with this parameter as the result may overflow.
+	#
+	# ~~~
+	# assert        1.to_bytes(2).hexdigest ==     "0001"
+	# assert    65535.to_bytes(2).hexdigest ==     "FFFF"
+	# assert     (-1).to_bytes(2).hexdigest ==     "FFFF"
+	# assert   (-512).to_bytes(4).hexdigest == "FFFFFE00"
+	# assert 0x123456.to_bytes(2).hexdigest ==     "3456"
+	# ~~~
+	#
 	# For 0, a Bytes object with single nul byte is returned (instead of an empty Bytes object).
 	#
 	# ~~~
 	# assert 0.to_bytes.hexdigest == "00"
 	# ~~~
 	#
-	# `Bytes::to_i` can be used to do the reverse operation.
+	# For positive integers, `Bytes::to_i` can reverse the operation.
 	#
 	# ~~~
 	# assert 1234.to_bytes.to_i == 1234
 	# ~~~
 	#
 	# Require self >= 0
-	fun to_bytes: Bytes do
-		if self == 0 then return "\0".to_bytes
-		assert self > 0
+	fun to_bytes(n_bytes: nullable Int): Bytes do
+
+		# If 0, force using at least one byte
+		if self == 0 and n_bytes == null then n_bytes = 1
 
 		# Compute the len (log256)
 		var len = 1
 		var max = 256
-		while self >= max do
+		var s = self.abs
+		while s >= max do
 			len += 1
 			max *= 256
 		end
 
+		# Two's complement
+		s = self
+		if self < 0 then
+			var ff = 0
+			for j in [0..len[ do
+				ff *= 0x100
+				ff += 0xFF
+			end
+
+			s = ((-self) ^ ff) + 1
+		end
+
+		# Cut long values
+		if n_bytes != null and len > n_bytes then len = n_bytes
+
 		# Allocate the buffer
-		var res = new Bytes.with_capacity(len)
-		for i in [0..len[ do res[i] = 0u8
+		var cap = n_bytes or else len
+		var res = new Bytes.with_capacity(cap)
+
+		var filler = if self < 0 then 0xFFu8 else 0u8
+		for i in [0..cap[ do res[i] = filler
 
 		# Fill it starting with the end
-		var i = len
-		var sum = self
-		while i > 0 do
+		var i = cap
+		var sum = s
+		while i > cap - len do
 			i -= 1
 			res[i] = (sum % 256).to_b
 			sum /= 256
 		end
+
 		return res
 	end
 end