Adding one-hot to binary helper function (#456)

* Adding one-hot to binary helper function

* Addressed comments for one-hot to binary helper function

pyrtl/__init__.py

@@ -40,6 +40,7 @@
 from .helperfuncs import find_and_print_loop
 from .helperfuncs import wire_struct
 from .helperfuncs import wire_matrix
+from .helperfuncs import one_hot_to_binary
 
 from .corecircuits import and_all_bits
 from .corecircuits import or_all_bits

pyrtl/helperfuncs.py

@@ -13,7 +13,7 @@
 from .core import working_block, _NameIndexer, _get_debug_mode, Block
 from .pyrtlexceptions import PyrtlError, PyrtlInternalError
 from .wire import WireVector, Input, Output, Const, Register, WrappedWireVector
-from .corecircuits import as_wires, rtl_all, rtl_any, concat, concat_list
+from .corecircuits import as_wires, rtl_all, rtl_any, concat, concat_list, select
 
 # -----------------------------------------------------------------
 #        ___       __   ___  __   __
@@ -1683,3 +1683,34 @@ def __len__(self):
             return len(self._components)
 
     return _WireMatrix
+
+
+def one_hot_to_binary(w) -> WireVector:
+    '''Takes a one-hot input and returns the bit position of the high bit in binary.
+
+    :param w: WireVector or a WireVector-like object or something that can be converted
+        into a Const (in accordance with the :py:func:`as_wires()` required input). Example
+        inputs: 0b0010, 64, 0b01.
+    :return: The bit position of the high bit in binary as a WireVector.
+
+    If the input contains multiple 1s, the bit position of the first 1 will
+        be returned. If the input contains no 1s, 0 will be returned.
+
+    Examples::
+
+        one_hot_to_binary(0b0010) # returns 1
+        one_hot_to_binary(64) # returns 6
+        one_hot_to_binary(0b1100) # returns 2, the bit position of the first 1
+        one_hot_to_binary(0) # returns 0
+    '''
+
+    w = as_wires(w)
+
+    pos = 0  # Bit position of the first 1
+    already_found = as_wires(False)  # True if first 1 already found, False otherwise
+
+    for i in range(len(w)):
+        pos = select(w[i] & ~already_found, i, pos)
+        already_found = already_found | w[i]
+
+    return pos

tests/test_helperfuncs.py

@@ -10,9 +10,9 @@
 import pyrtl.helperfuncs
 from pyrtl.rtllib import testingutils as utils
 
-
 # ---------------------------------------------------------------
 
+
 class TestWireVectorList(unittest.TestCase):
     def setUp(self):
         pass
@@ -1772,5 +1772,47 @@ def test_byte_matrix_input_concatenate(self):
         self.assertEqual(sim.inspect('byte_matrix[0].low'), 0xB)
 
 
+class TestOneHotToBinary(unittest.TestCase):
+    def setUp(self):
+        pyrtl.reset_working_block()
+
+    def test_simple_onehot(self):
+        i = pyrtl.Input(bitwidth=8, name='i')
+        o = pyrtl.Output(bitwidth=3, name='o')
+        o <<= pyrtl.one_hot_to_binary(i)
+
+        sim = pyrtl.Simulation()
+        sim.step({i: 0b00000001})
+        self.assertEqual(sim.inspect('o'), 0)
+        sim.step({i: 0b10000000})
+        self.assertEqual(sim.inspect('o'), 7)
+        sim.step({i: 32})
+        self.assertEqual(sim.inspect('o'), 5)
+        sim.step({i: 16})
+        self.assertEqual(sim.inspect('o'), 4)
+
+    def test_multiple_ones(self):
+        i = pyrtl.Input(bitwidth=8, name='i')
+        o = pyrtl.Output(bitwidth=3, name='o')
+        o <<= pyrtl.one_hot_to_binary(i)
+
+        sim = pyrtl.Simulation()
+        sim.step({i: 0b00000101})
+        self.assertEqual(sim.inspect('o'), 0)
+        sim.step({i: 0b11000000})
+        self.assertEqual(sim.inspect('o'), 6)
+        sim.step({i: 0b10010010})
+        self.assertEqual(sim.inspect('o'), 1)
+
+    def test_no_ones(self):
+        i = pyrtl.Input(bitwidth=8, name='i')
+        o = pyrtl.Output(bitwidth=3, name='o')
+        o <<= pyrtl.one_hot_to_binary(i)
+
+        sim = pyrtl.Simulation()
+        sim.step({i: 0b00000000})
+        self.assertEqual(sim.inspect('o'), 0)
+
+
 if __name__ == "__main__":
     unittest.main()