core: `Bytes::to_i` can parse signed ints

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

diff --git a/lib/core/bytes.nit b/lib/core/bytes.nit
index 6ef4a90..84276ac 100644 (file)
--- a/lib/core/bytes.nit
+++ b/lib/core/bytes.nit
@@ -328,35 +328,48 @@ class Bytes
                return new FlatString.full(ns, elen, 0, elen)
        end
 
-       # Interprets `self` as a big-endian positive integer.
+       # Interprets `self` as a big-endian integer (unsigned by default)
        #
        # ~~~
        # var b = "0102".hexdigest_to_bytes
        # assert b.to_i == 258
+       #
+       # assert   "01".hexdigest_to_bytes.to_i == 1
+       # assert   "FF".hexdigest_to_bytes.to_i == 255
+       # assert "0000".hexdigest_to_bytes.to_i == 0
        # ~~~
        #
-       # Nul bytes on the left are trimmed.
-       # 0 is returned for an empty Bytes object.
+       # If `self.is_empty`, 0 is returned.
        #
        # ~~~
-       # assert "01".hexdigest_to_bytes.to_i == 1
-       # assert "0001".hexdigest_to_bytes.to_i == 1
-       #
-       # assert "0000".hexdigest_to_bytes.to_i == 0
-       # assert "00".hexdigest_to_bytes.to_i == 0
        # assert "".hexdigest_to_bytes.to_i == 0
        # ~~~
        #
+       # If `signed == true`, the bytes are read as a signed integer.
+       # As usual, the sign bit is the left most bit, no matter the
+       # `length` of `self`.
+       #
+       # ~~~
+       # assert     "01".hexdigest_to_bytes.to_i(true) ==      1
+       # assert     "FF".hexdigest_to_bytes.to_i(true) ==     -1
+       # assert   "00FF".hexdigest_to_bytes.to_i(true) ==    255
+       # assert     "E0".hexdigest_to_bytes.to_i(true) ==    -32
+       # assert   "FE00".hexdigest_to_bytes.to_i(true) ==   -512
+       # assert "FEFEFE".hexdigest_to_bytes.to_i(true) == -65794
+       # ~~~
+       #
        # `Int::to_bytes` is a loosely reverse method.
        #
        # ~~~
        # assert b.to_i.to_bytes == b
        # assert (b.to_i + 1).to_bytes.hexdigest == "0103"
        # assert "0001".hexdigest_to_bytes.to_i.to_bytes.hexdigest == "01"
+       #
+       # assert (-32).to_bytes.to_i(true) == -32
        # ~~~
        #
        # Warning: `Int` might overflow for bytes with more than 60 bits.
-       fun to_i: Int do
+       fun to_i(signed: nullable Bool): Int do
                var res = 0
                var i = 0
                while i < length do
@@ -364,6 +377,18 @@ class Bytes
                        res += self[i].to_i
                        i += 1
                end
+
+               # Two's complement is `signed`
+               if signed == true and not_empty and first > 0x80u8 then
+                       var ff = 0
+                       for j in [0..length[ do
+                               ff *= 0x100
+                               ff += 0xFF
+                       end
+
+                       res = -((res ^ ff) + 1)
+               end
+
                return res
        end