From 38ce9d0abbb4b72d437c3fbe1fbbfa3c132ff252 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Sat, 26 Nov 2022 01:12:11 +0000
Subject: [PATCH] Add step to solve() to check if cells have only one possible
 value

---
 solve.c | 22 +++++++++++++++++++++-
 sud.h   |  1 +
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/solve.c b/solve.c
index 6e6db06..ba9c90a 100644
--- a/solve.c
+++ b/solve.c
@@ -24,7 +24,7 @@ int solve(struct sudoku *sud)
 {
 	unsigned r, c, n, i, j, val, np, pr, pc;
 	bool match;
-	uint16_t valmask;
+	uint16_t valmask, pvals;
 
 	for (n = 0;; ++n) {
 		match = false;
@@ -103,6 +103,26 @@ int solve(struct sudoku *sud)
 			}
 		}
 
+		/* Check each cell for if it has only one possible value. */
+		for (r = 0; r < NDIGITS; ++r) {
+			for (c = 0; c < NDIGITS; ++c) {
+				if (DET(sud->cells[r][c]))
+					continue;
+
+				pvals = sud->cells[r][c] & PVALSMASK;
+				for (val = 0; val < NDIGITS; ++val) {
+					if (pvals & 1) {
+						if (pvals == 1) {
+							update(sud, r, c, val);
+							match = true;
+						}
+						break;
+					}
+					pvals >>= 1;
+				}
+			}
+		}
+
 		/* Exit if no matches. */
 		if (!match)
 			return n;
diff --git a/sud.h b/sud.h
index e9cf138..3ced19d 100644
--- a/sud.h
+++ b/sud.h
@@ -29,6 +29,7 @@
 #define DETMASK 0x8000
 #define VALSHIFT 9
 #define VALMASK 0x1e00
+#define PVALSMASK 0x01ff
 
 #define DET(x) (x & DETMASK)
 #define VAL(x) ((x & VALMASK) >> VALSHIFT)