From 0ca01985cd80dcdbca0c189d55bab73442ab43f4 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Wed, 23 Nov 2022 21:41:59 +0000
Subject: [PATCH] Add steps to test columns and segments back

Turns out these are needed for some of them... I guess the way I was
generating them was somehow avoiding tricky ones being created?
---
 solve.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/solve.c b/solve.c
index a0d1420..9bda67f 100644
--- a/solve.c
+++ b/solve.c
@@ -22,7 +22,7 @@
 
 int solve(struct sudoku *sud)
 {
-	unsigned r, c, n, val, np, pr, pc;
+	unsigned r, c, n, i, j, val, np, pr, pc;
 	bool match;
 
 	for (n = 0;; ++n) {
@@ -41,7 +41,50 @@ int solve(struct sudoku *sud)
 							break;
 					}
 				}
+				if (np == 1) {
+					if (update(sud, pr, pc, val) != OK)
+						return -1;
+					match = true;
+				}
+			}
+		}
 
+		/* Check if only one place a value can go on each column. */
+		for (c = 0; c < NDIGITS; ++c) {
+			for (val = 0; val < NDIGITS; ++val) {
+				np = 0;
+				for (r = 0; r < NDIGITS; ++r) {
+					if (!sud->cells[r][c].det && sud->cells[r][c].pvals[val]) {
+						pr = r;
+						pc = c;
+						++np;
+						if (np > 1)
+							break;
+					}
+				}
+				if (np == 1) {
+					if (update(sud, pr, pc, val) != OK)
+						return -1;
+					match = true;
+				}
+			}
+		}
+
+		/* Check if only one place a value can go in each segment. */
+		for (i = 0; i < NDIGITS; ++i) {
+			for (val = 0; val < NDIGITS; ++val) {
+				np = 0;
+				for (j = 0; j < NDIGITS; ++j) {
+					r = SEGLEN * (i / SEGLEN) + j / SEGLEN;
+					c = SEGLEN * (i % SEGLEN) + j % SEGLEN;
+					if (!sud->cells[r][c].det && sud->cells[r][c].pvals[val]) {
+						pr = r;
+						pc = c;
+						++np;
+						if (np > 1)
+							break;
+					}
+				}
 				if (np == 1) {
 					if (update(sud, pr, pc, val) != OK)
 						return -1;