From 070fa146ecbc9c303e821c64bfad5805008b0c07 Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Fri, 3 May 2019 15:35:23 -0700
Subject: [PATCH] internal/scalar: add constant-time signed radix 16

Closes #10
---
 internal/scalar/scalar.go | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/internal/scalar/scalar.go b/internal/scalar/scalar.go
index 59b5701..18f4767 100644
--- a/internal/scalar/scalar.go
+++ b/internal/scalar/scalar.go
@@ -958,3 +958,22 @@ func (s *Scalar) NonAdjacentForm(w uint) [256]int8 {
 	}
 	return naf
 }
+
+func (s *Scalar) SignedRadix16() [64]int8 {
+	var digits [64]int8
+
+	// Compute unsigned radix-16 digits:
+	for i := 0; i < 32; i++ {
+		digits[2*i] = i8(s[i]) & 15
+		digits[2*i+1] = i8(s[i]>>4) & 15
+	}
+
+	// Recenter coefficients:
+	for i := 0; i < 63; i++ {
+		carry := (digits[i] + 8) >> 4
+		digits[i] -= carry << 4
+		digits[i+1] += carry
+	}
+
+	return digits
+}