Improved task 3448

Author: javadev

src/main/java/g3401_3500/s3448_count_substrings_divisible_by_last_digit/Solution.java

@@ -5,63 +5,103 @@
 public class Solution {
     public long countSubstrings(String s) {
         int n = s.length();
-        long ans = 0;
         int[] p3 = new int[n];
         int[] p7 = new int[n];
         int[] p9 = new int[n];
+        computeModArrays(s, p3, p7, p9);
+        long[] freq3 = new long[3];
+        long[] freq9 = new long[9];
+        long[][] freq7 = new long[6][7];
+        int[] inv7 = {1, 5, 4, 6, 2, 3};
+        return countValidSubstrings(s, p3, p7, p9, freq3, freq9, freq7, inv7);
+    }
+
+    private void computeModArrays(String s, int[] p3, int[] p7, int[] p9) {
         p3[0] = (s.charAt(0) - '0') % 3;
         p7[0] = (s.charAt(0) - '0') % 7;
         p9[0] = (s.charAt(0) - '0') % 9;
-        for (int i = 1; i < n; i++) {
+        for (int i = 1; i < s.length(); i++) {
             int dig = s.charAt(i) - '0';
             p3[i] = (p3[i - 1] * 10 + dig) % 3;
             p7[i] = (p7[i - 1] * 10 + dig) % 7;
             p9[i] = (p9[i - 1] * 10 + dig) % 9;
         }
-        long[] freq3 = new long[3];
-        long[] freq9 = new long[9];
-        long[][] freq7 = new long[6][7];
-        int[] inv7 = {1, 5, 4, 6, 2, 3};
-        for (int j = 0; j < n; j++) {
+    }
+
+    private long countValidSubstrings(
+            String s,
+            int[] p3,
+            int[] p7,
+            int[] p9,
+            long[] freq3,
+            long[] freq9,
+            long[][] freq7,
+            int[] inv7) {
+        long ans = 0;
+        for (int j = 0; j < s.length(); j++) {
             int d = s.charAt(j) - '0';
             if (d != 0) {
-                if (d == 1 || d == 2 || d == 5) {
-                    ans += (j + 1);
-                } else if (d == 4) {
-                    if (j == 0) {
-                        ans += 1;
-                    } else {
-                        int num = (s.charAt(j - 1) - '0') * 10 + d;
-                        ans += (num % 4 == 0 ? (j + 1) : 1);
-                    }
-                } else if (d == 8) {
-                    if (j == 0) {
-                        ans += 1;
-                    } else if (j == 1) {
-                        int num = (s.charAt(0) - '0') * 10 + 8;
-                        ans += (num % 8 == 0 ? 2 : 1);
-                    } else {
-                        int num3 = (s.charAt(j - 2) - '0') * 100 + (s.charAt(j - 1) - '0') * 10 + 8;
-                        int num2 = (s.charAt(j - 1) - '0') * 10 + 8;
-                        ans += ((num3 % 8 == 0 ? (j - 1) : 0) + (num2 % 8 == 0 ? 1 : 0) + 1);
-                    }
-                } else if (d == 3 || d == 6) {
-                    ans += (p3[j] == 0 ? 1L : 0L) + freq3[p3[j]];
-                } else if (d == 7) {
-                    ans += (p7[j] == 0 ? 1L : 0L);
-                    for (int m = 0; m < 6; m++) {
-                        int idx = ((j % 6) - m + 6) % 6;
-                        int req = (p7[j] * inv7[m]) % 7;
-                        ans += freq7[idx][req];
-                    }
-                } else if (d == 9) {
-                    ans += (p9[j] == 0 ? 1L : 0L) + freq9[p9[j]];
-                }
+                ans += countDivisibilityCases(s, j, d, p3, p7, p9, freq3, freq9, freq7, inv7);
             }
             freq3[p3[j]]++;
             freq7[j % 6][p7[j]]++;
             freq9[p9[j]]++;
         }
         return ans;
     }
+
+    private long countDivisibilityCases(
+            String s,
+            int j,
+            int d,
+            int[] p3,
+            int[] p7,
+            int[] p9,
+            long[] freq3,
+            long[] freq9,
+            long[][] freq7,
+            int[] inv7) {
+        long ans = 0;
+        if (d == 1 || d == 2 || d == 5) {
+            ans += (j + 1);
+        } else if (d == 4) {
+            ans += countDivisibilityBy4(s, j);
+        } else if (d == 8) {
+            ans += countDivisibilityBy8(s, j);
+        } else if (d == 3 || d == 6) {
+            ans += (p3[j] == 0 ? 1L : 0L) + freq3[p3[j]];
+        } else if (d == 7) {
+            ans += countDivisibilityBy7(j, p7, freq7, inv7);
+        } else if (d == 9) {
+            ans += (p9[j] == 0 ? 1L : 0L) + freq9[p9[j]];
+        }
+        return ans;
+    }
+
+    private long countDivisibilityBy4(String s, int j) {
+        if (j == 0) return 1;
+        int num = (s.charAt(j - 1) - '0') * 10 + (s.charAt(j) - '0');
+        return num % 4 == 0 ? j + 1 : 1;
+    }
+
+    private long countDivisibilityBy8(String s, int j) {
+        if (j == 0) return 1;
+        if (j == 1) {
+            int num = (s.charAt(0) - '0') * 10 + 8;
+            return (num % 8 == 0 ? 2 : 1);
+        }
+        int num3 = (s.charAt(j - 2) - '0') * 100 + (s.charAt(j - 1) - '0') * 10 + 8;
+        int num2 = (s.charAt(j - 1) - '0') * 10 + 8;
+        return (num3 % 8 == 0 ? j - 1 : 0) + (num2 % 8 == 0 ? 1 : 0) + 1;
+    }
+
+    private long countDivisibilityBy7(int j, int[] p7, long[][] freq7, int[] inv7) {
+        long ans = (p7[j] == 0 ? 1L : 0L);
+        for (int m = 0; m < 6; m++) {
+            int idx = ((j % 6) - m + 6) % 6;
+            int req = (p7[j] * inv7[m]) % 7;
+            ans += freq7[idx][req];
+        }
+        return ans;
+    }
 }