diff --git a/.gitignore b/.gitignore
index d392cc8..74e1b75 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 *.o
 sudoku
 eval
+genlut
 
 # Test data
 sudoku.csv
@@ -15,3 +16,6 @@ compile_commands.json
 
 # Profiling results
 gmon.out
+
+# Generated code
+lut.c
diff --git a/Makefile b/Makefile
index 47b33ea..17817e2 100644
--- a/Makefile
+++ b/Makefile
@@ -24,10 +24,7 @@ CFLAGS += -static
 
 LDFLAGS += -lpthread
 
-# For profiling
-# CFLAGS += -pg
-
-SRC = main.c sud.c solve.c
+SRC = main.c sud.c solve.c lut.c
 OBJ = $(SRC:.c=.o)
 
 default: sudoku eval
@@ -35,16 +32,16 @@ default: sudoku eval
 sudoku: $(OBJ)
 	$(CC) $(CFLAGS) $(LDFLAGS) $(OBJ) -o $@
 
-eval: eval.o
-	$(CC) $(CFLAGS) $(LDFLAGS) eval.o -o $@
-
 clean:
-	rm -f sudoku *.o
+	rm -f sudoku eval genlut *.o lut.c
+
+lut.c: genlut
+	./genlut > $@
 
 eval.o: ds.h
 main.o: sud.h solve.h ds.h
-sud.o: sud.h
-solve.o: solve.h
+sud.o: sud.h lut.h
+solve.o: solve.h lut.h
 
 puzzles: sudoku.csv
 	pv sudoku.csv | tail -n+2 | cut -d, -f1 | tr -d '\n' > $@
diff --git a/genlut.c b/genlut.c
new file mode 100644
index 0000000..968203f
--- /dev/null
+++ b/genlut.c
@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2022 Camden Dixie O'Brien
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public
+ * License along with this program.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#include "sud.h"
+
+#include <stdio.h>
+
+#define Q(x) #x
+#define PRLUT(x) prlut(Q(x), x)
+
+static void prlut(const char *name, unsigned lut[NCELLS][NDIGITS - 1])
+{
+	printf("const unsigned %s[NCELLS][NDIGITS - 1] = {\n", name);
+	for (unsigned i = 0; i < NCELLS; ++i) {
+		printf("\t[%2u] = { ", i);
+		for (unsigned j = 0; j < NDIGITS - 1; ++j) {
+			printf("%2u", lut[i][j]);
+			if (j != NDIGITS - 2)
+				putchar(',');
+			putchar(' ');
+		}
+		fputs("},\n", stdout);
+	}
+	fputs("};\n", stdout);
+}
+
+int main(void)
+{
+	unsigned rowidx_lut[NCELLS][NDIGITS - 1];
+	unsigned colidx_lut[NCELLS][NDIGITS - 1];
+	unsigned segidx_lut[NCELLS][NDIGITS - 1];
+
+	/* Populate tables. */
+	unsigned rowi, coli, segi;
+	for (unsigned r = 0; r < NDIGITS; ++r) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
+			/* Calculate row and column indices. */
+			rowi = coli = 0;
+			for (unsigned i = 0; i < NDIGITS; ++i) {
+				if (i != c)
+					rowidx_lut[r * NDIGITS + c][rowi++] = r * NDIGITS + i;
+				if (i != r)
+					colidx_lut[r * NDIGITS + c][coli++] = i * NDIGITS + c;
+			}
+
+			/* Calculate segment indices. */
+			segi = 0;
+			const unsigned sr = SEGLEN * (r / SEGLEN);
+			const unsigned sc = SEGLEN * (c / SEGLEN);
+			for (unsigned i = sr; i < sr + SEGLEN; ++i) {
+				for (unsigned j = sc; j < sc + SEGLEN; ++j) {
+					if (i == r && j == c)
+						continue;
+					segidx_lut[r * NDIGITS + c][segi++] = i * NDIGITS + j;
+				}
+			}
+		}
+	}
+
+	printf("/*\n * This file is auto-generated; do not edit.\n */\n\n");
+	printf("#include \"lut.h\"\n\n");
+	PRLUT(rowidx_lut);
+	putchar('\n');
+	PRLUT(colidx_lut);
+	putchar('\n');
+	PRLUT(segidx_lut);
+
+	return 0;
+}
diff --git a/lut.h b/lut.h
new file mode 100644
index 0000000..603fdeb
--- /dev/null
+++ b/lut.h
@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2022 Camden Dixie O'Brien
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public
+ * License along with this program.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef LUT_H
+#define LUT_H
+
+#include "sud.h"
+
+#define NGROUP (NDIGITS - 1)
+
+extern const unsigned rowidx_lut[NCELLS][NGROUP];
+extern const unsigned colidx_lut[NCELLS][NGROUP];
+extern const unsigned segidx_lut[NCELLS][NGROUP];
+
+#endif
diff --git a/solve.c b/solve.c
index f17ae60..b5cf04d 100644
--- a/solve.c
+++ b/solve.c
@@ -17,14 +17,15 @@
  */
 
 #include "solve.h"
+#include "lut.h"
 
 #include <stdio.h>
 
-static void setpval(struct sudoku *sud, unsigned r, unsigned c, uint16_t pval)
+static void setpval(struct sudoku *sud, unsigned i, uint16_t pval)
 {
 	for (unsigned val = 0; val < NDIGITS; ++val) {
 		if (pval & 1) {
-			update(sud, r, c, val);
+			update(sud, i, val);
 			return;
 		}
 		pval >>= 1;
@@ -33,86 +34,80 @@ static void setpval(struct sudoku *sud, unsigned r, unsigned c, uint16_t pval)
 
 int solve(struct sudoku *sud)
 {
-	unsigned r, c, n, i, j, val, sr, sc;
+	unsigned n, i, j, val;
 	bool match;
 	uint16_t valmask, pvals;
 
 	for (n = 0;; ++n) {
 		match = false;
 
-		for (r = 0; r < NDIGITS; ++r) {
-			for (c = 0; c < NDIGITS; ++c) {
-				if (DET(sud->cells[r][c]))
-					continue;
+		for (i = 0; i < NCELLS; ++i) {
+			if (DET(sud->cells[i]))
+				continue;
 
-				/*
-				 * Check if there's only one possible value this cell
-				 * can have.
-				 */
-				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;
-							goto next_cell;
-						}
-						break;
+			/*
+			 * Check if there's only one possible value this cell
+			 * can have.
+			 */
+			pvals = sud->cells[i] & PVALSMASK;
+			for (val = 0; val < NDIGITS; ++val) {
+				if (pvals & 1) {
+					if (pvals == 1) {
+						update(sud, i, val);
+						match = true;
+						goto next_cell;
 					}
-					pvals >>= 1;
+					break;
 				}
-
-				/*
-				 * Check if there's a possible value unique to this
-				 * cell in its row.
-				 */
-				valmask = 0;
-				for (i = 0; i < NDIGITS; ++i) {
-					if (i != r && !DET(sud->cells[i][c]))
-						valmask |= sud->cells[i][c];
-				}
-				if ((pvals = sud->cells[r][c] & ~valmask)) {
-					setpval(sud, r, c, pvals);
-					match = true;
-					continue;
-				}
-
-				/*
-				 * Check if there's a possible value unique to this
-				 * cell in its column.
-				 */
-				valmask = 0;
-				for (i = 0; i < NDIGITS; ++i) {
-					if (i != c && !DET(sud->cells[r][i]))
-						valmask |= sud->cells[r][i];
-				}
-				if ((pvals = sud->cells[r][c] & ~valmask)) {
-					setpval(sud, r, c, pvals);
-					match = true;
-					continue;
-				}
-
-				/*
-				 * Check if there's a possible value unique to this
-				 * cell in its segment.
-				 */
-				sr = SEGLEN * (r / SEGLEN);
-				sc = SEGLEN * (c / SEGLEN);
-				valmask = 0;
-				for (i = sr; i < sr + SEGLEN; ++i) {
-					for (j = sc; j < sc + SEGLEN; ++j) {
-						if ((i != r || j != c) && !DET(sud->cells[i][j]))
-							valmask |= sud->cells[i][j];
-					}
-				}
-				if ((pvals = sud->cells[r][c] & ~valmask)) {
-					setpval(sud, r, c, pvals);
-					match = true;
-					continue;
-				}
-
-			next_cell:;
+				pvals >>= 1;
 			}
+
+			/*
+			 * Check if there's a possible value unique to this
+			 * cell in its row.
+			 */
+			valmask = 0;
+			for (j = 0; j < NGROUP; ++j) {
+				if (!DET(sud->cells[rowidx_lut[i][j]]))
+					valmask |= sud->cells[rowidx_lut[i][j]];
+			}
+			if ((pvals = sud->cells[i] & ~valmask)) {
+				setpval(sud, i, pvals);
+				match = true;
+				continue;
+			}
+
+			/*
+			 * Check if there's a possible value unique to this
+			 * cell in its column.
+			 */
+			valmask = 0;
+			for (j = 0; j < NGROUP; ++j) {
+				if (!DET(sud->cells[colidx_lut[i][j]]))
+					valmask |= sud->cells[colidx_lut[i][j]];
+			}
+			if ((pvals = sud->cells[i] & ~valmask)) {
+				setpval(sud, i, pvals);
+				match = true;
+				continue;
+			}
+
+			/*
+			 * Check if there's a possible value unique to this
+			 * cell in its segment.
+			 */
+			valmask = 0;
+			for (j = 0; j < NGROUP; ++j) {
+				if (!DET(sud->cells[segidx_lut[i][j]]))
+					valmask |= sud->cells[segidx_lut[i][j]];
+			}
+			if ((pvals = sud->cells[i] & ~valmask)) {
+				setpval(sud, i, pvals);
+				match = true;
+				continue;
+			}
+
+		next_cell:;
 		}
 
 		/* Exit if no matches. */
diff --git a/sud.c b/sud.c
index be0ac41..51c8a21 100644
--- a/sud.c
+++ b/sud.c
@@ -16,6 +16,7 @@
  * <https://www.gnu.org/licenses/>.
  */
 
+#include "lut.h"
 #include "sud.h"
 
 #include <stdio.h>
@@ -27,10 +28,8 @@
 
 static void init(struct sudoku *sud)
 {
-	for (unsigned r = 0; r < NDIGITS; ++r) {
-		for (unsigned c = 0; c < NDIGITS; ++c)
-			sud->cells[r][c] = CELLINIT;
-	}
+	for (unsigned i = 0; i < NCELLS; ++i)
+		sud->cells[i] = CELLINIT;
 }
 
 bool load(struct sudoku *sud, const char *ptr)
@@ -41,7 +40,7 @@ bool load(struct sudoku *sud, const char *ptr)
 	for (unsigned i = 0; i < NCELLS; ++i) {
 		if (ptr[i] == '0')
 			continue;
-		if (update(sud, i / NDIGITS, i % NDIGITS, ptr[i] - '1') != OK)
+		if (update(sud, i, ptr[i] - '1') != OK)
 			return false;
 	}
 
@@ -50,40 +49,29 @@ bool load(struct sudoku *sud, const char *ptr)
 
 void save(struct sudoku *sud, char *ptr)
 {
-	for (unsigned r = 0; r < NDIGITS; ++r) {
-		for (unsigned c = 0; c < NDIGITS; ++c) {
-			if (DET(sud->cells[r][c]))
-				*ptr++ = '1' + VAL(sud->cells[r][c]);
-			else
-				*ptr++ = '0';
-		}
+	for (unsigned i = 0; i < NCELLS; ++i) {
+		if (DET(sud->cells[i]))
+			*ptr++ = '1' + VAL(sud->cells[i]);
+		else
+			*ptr++ = '0';
 	}
 }
 
-enum update_res update(struct sudoku *sud, unsigned r, unsigned c, unsigned val)
+enum update_res update(struct sudoku *sud, unsigned i, unsigned val)
 {
-	unsigned tr, tc;
 	const uint16_t clearmask = ~(1 << val);
 
-	/* Update possible values of cells in same row. */
-	for (tc = 0; tc < NDIGITS; ++tc)
-		sud->cells[r][tc] &= clearmask;
-
-	/* Update possible values of cells in same column. */
-	for (tr = 0; tr < NDIGITS; ++tr)
-		sud->cells[tr][c] &= clearmask;
-
-	/* Update possible values of cells in same segment. */
-	const unsigned segr0 = SEGLEN * (r / SEGLEN);
-	const unsigned segc0 = SEGLEN * (c / SEGLEN);
-	for (tr = segr0; tr < segr0 + SEGLEN; ++tr) {
-		for (tc = segc0; tc < segc0 + SEGLEN; ++tc)
-			sud->cells[tr][tc] &= clearmask;
+	/* Update possible values of cells in same row, column and
+	 * segment. */
+	for (unsigned j = 0; j < NGROUP; ++j) {
+		sud->cells[rowidx_lut[i][j]] &= clearmask;
+		sud->cells[colidx_lut[i][j]] &= clearmask;
+		sud->cells[segidx_lut[i][j]] &= clearmask;
 	}
 
 	/* Set the cell's value. */
-	sud->cells[r][c] |= DETMASK;
-	sud->cells[r][c] |= val << VALSHIFT;
+	sud->cells[i] |= DETMASK;
+	sud->cells[i] |= val << VALSHIFT;
 
 	return OK;
 }
@@ -106,8 +94,8 @@ void print(const struct sudoku *sud)
 			if (c != 0 && c % SEGLEN == 0)
 				fputs("| ", stdout);
 
-			if (DET(sud->cells[r][c]))
-				printf("%u ", VAL(sud->cells[r][c]) + 1);
+			if (DET(sud->cells[IDX(r, c)]))
+				printf("%u ", VAL(sud->cells[IDX(r, c)]) + 1);
 			else
 				fputs("  ", stdout);
 		}
@@ -138,9 +126,9 @@ enum check_res check(const struct sudoku *sud)
 	for (r = 0; r < NDIGITS; ++r) {
 		zerocounts(digitcounts);
 		for (c = 0; c < NDIGITS; ++c) {
-			if (!DET(sud->cells[r][c]))
+			if (!DET(sud->cells[IDX(r, c)]))
 				return INCOMPLETE;
-			++digitcounts[VAL(sud->cells[r][c])];
+			++digitcounts[VAL(sud->cells[IDX(r, c)])];
 		}
 		if (!checkcounts(digitcounts))
 			return INCORRECT;
@@ -150,9 +138,9 @@ enum check_res check(const struct sudoku *sud)
 	for (c = 0; c < NDIGITS; ++c) {
 		zerocounts(digitcounts);
 		for (r = 0; r < NDIGITS; ++r) {
-			if (!DET(sud->cells[r][c]))
+			if (!DET(sud->cells[IDX(r, c)]))
 				return INCOMPLETE;
-			++digitcounts[VAL(sud->cells[r][c])];
+			++digitcounts[VAL(sud->cells[IDX(r, c)])];
 		}
 		if (!checkcounts(digitcounts))
 			return INCORRECT;
@@ -164,9 +152,9 @@ enum check_res check(const struct sudoku *sud)
 		for (j = 0; j < NDIGITS; ++j) {
 			r = 3 * (i / 3) + j / 3;
 			c = 3 * (i % 3) + j % 3;
-			if (!DET(sud->cells[r][c]))
+			if (!DET(sud->cells[IDX(r, c)]))
 				return INCOMPLETE;
-			++digitcounts[VAL(sud->cells[r][c])];
+			++digitcounts[VAL(sud->cells[IDX(r, c)])];
 		}
 		if (!checkcounts(digitcounts))
 			return INCORRECT;
@@ -180,7 +168,7 @@ bool filled(const struct sudoku *sud)
 {
 	for (unsigned r = 0; r < NDIGITS; ++r) {
 		for (unsigned c = 0; c < NDIGITS; ++c) {
-			if (!(DET(sud->cells[r][c])))
+			if (!(DET(sud->cells[IDX(r, c)])))
 				return false;
 		}
 	}
diff --git a/sud.h b/sud.h
index 3ced19d..d11f411 100644
--- a/sud.h
+++ b/sud.h
@@ -34,8 +34,10 @@
 #define DET(x) (x & DETMASK)
 #define VAL(x) ((x & VALMASK) >> VALSHIFT)
 
+#define IDX(r, c) (NDIGITS * (r) + (c))
+
 struct sudoku {
-	uint16_t cells[NDIGITS][NDIGITS];
+	uint16_t cells[NCELLS];
 };
 
 enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
@@ -54,13 +56,10 @@ bool load(struct sudoku *sud, const char *ptr);
 void save(struct sudoku *sud, char *ptr);
 
 /**
- * Attempt to update the cell at row `r`, column `c` to have the value
- * `val`. Returns `OK` on success, `ALREADY_DET` if the cell is
- * already determined or `NOT_ALLOWED` if the cell being `val` would
- * violate the sudoku rules.
+ * Attempt to update the cell at index `i` to have the value
+ * `val`. Returns `OK`.
  */
-enum update_res
-update(struct sudoku *sud, unsigned r, unsigned c, unsigned val);
+enum update_res update(struct sudoku *sud, unsigned i, unsigned val);
 
 /**
  * Print a string representation of the sudoku to stdout.