Array element selection (index) circuit.

compiler/circuits/circ_index.go

@@ -0,0 +1,84 @@
+//
+// Copyright (c) 2021 Markku Rossi
+//
+// All rights reserved.
+//
+
+package circuits
+
+import (
+	"fmt"
+)
+
+// NewIndex creates a new array element selection (index) circuit.
+func NewIndex(compiler *Compiler, size int, array, index, out []*Wire) error {
+	if len(array)%size != 0 {
+		return fmt.Errorf("array width %d must be multiple of element size %d",
+			len(array), size)
+	}
+	if len(out) < size {
+		return fmt.Errorf("out %d too small for element size %d",
+			len(out), size)
+	}
+	n := len(array) / size
+	if n == 0 {
+		for i := 0; i < len(out); i++ {
+			out[i] = compiler.ZeroWire()
+		}
+		return nil
+	}
+	selected := make([]*Wire, size)
+	for i := 0; i < size; i++ {
+		selected[i] = compiler.ZeroWire()
+	}
+
+	// Number of bits needed for indices.
+	var bits int
+	for bits = 1; bits < size; bits *= 2 {
+	}
+	fmt.Printf("selecting with %d bits\n", bits)
+
+	if len(index) > bits {
+		index = index[0:bits]
+	}
+
+	for n--; n >= 0; n-- {
+		// Compare index to n, result to cond
+		nWires := make([]*Wire, bits)
+		for i := 0; i < bits; i++ {
+			if n&(1<<i) == 0 {
+				nWires[i] = compiler.ZeroWire()
+			} else {
+				nWires[i] = compiler.OneWire()
+			}
+		}
+		nWires, index = compiler.ZeroPad(nWires, index)
+
+		cond := make([]*Wire, 1)
+		cond[0] = NewWire()
+		err := NewEqComparator(compiler, nWires, index, cond)
+		if err != nil {
+			return err
+		}
+
+		// MUX cond, array[n*size:(n+1)*size], selected
+
+		var result []*Wire
+		if n == 0 {
+			result = out
+		} else {
+			result = make([]*Wire, size)
+			for i := 0; i < size; i++ {
+				result[i] = NewWire()
+			}
+		}
+		err = NewMUX(compiler, cond, array[n*size:(n+1)*size], selected, result)
+		if err != nil {
+			return err
+		}
+
+		selected = result
+	}
+
+	return nil
+}