add a pushFront operation for the bitranges

ranges/bitranges.nim

@@ -174,6 +174,12 @@ proc setAbsoluteBit(x: BitRange, absIdx: int, val: bool) {.inline.} =
   if val:
     raiseBitBE x.data[byteToWrite], bitToWrite
 
+proc pushFront*(x: var BitRange, val: bool) =
+  assert x.start > 0
+  dec x.start
+  x[0] = val
+  inc x.mLen
+
 template neededBytes(nBits: int): int =
   (nBits shr 3) + ord((nBits and 0b111) != 0)
 

tests/tbitranges.nim

@@ -68,3 +68,16 @@ suite "bir ranges":
       for idx, bit in x:
         y[idx] = bit
       check x == y
+
+  test "constructor with start":
+    var a = @[byte 0b10101010, 0b11110000, 0b00001111, 0b01010101]
+    var b = a.bits(1, 8)
+    check b.len == 8
+    check b[0] == false
+    check $b == "01010101"
+    b[0] = true
+    check $b == "11010101"
+    check b[0] == true
+    b.pushFront(false)
+    check b[0] == false
+    check $b == "011010101"