🧑‍💻 improves hash range query implementation

algorithms/strings/hash-range-query.cpp

@@ -1,28 +1,36 @@
+const ll P = 31;
+const ll MOD = 1e9 + 9;
+const int MAXN(1e6);
+
+ll ppow[MAXN+1];
+void pre_calc() {
+	ppow[0] = 1;
+	for (int i = 1; i <= MAXN; i++)
+		ppow[i] = (ppow[i-1] * P) % MOD;
+}
+
 struct Hash {
-  const ll P = 31;
-  const ll mod = 1e9 + 7;
-  string s;
   int n;
-  vll h, hi, p;
-  Hash() {}
-  Hash(string s) : s(s), n(s.size()), h(n), hi(n), p(n) {
-    for (int i = 0; i < n; i++)
-      p[i] = (i ? P * p[i - 1] : 1) % mod;
-    for (int i = 0; i < n; i++)
-      h[i] = (s[i] + (i ? h[i - 1] : 0) * P) % mod;
-    for (int i = n - 1; i >= 0; i--)
-      hi[i] =
-        (s[i] + (i + 1 < n ? hi[i + 1] : 0) * P) % mod;
+  vll h, hi;
+  Hash(const string &s) : n(s.size()), h(n), hi(n) {
+    h[0] = s[0];
+    hi[n-1] = s[n-1];
+    for (int i = 1; i < n ; i++) {
+      h[i] = (s[i] +  h[i - 1] * P) % MOD;
+      hi[n-i-1] = (s[n-i-1] + hi[n - i - 1] * P) % MOD;
+    }
   }
-  ll query(int l, int r) {
+
+  ll qry(int l, int r) {
     ll hash =
-      (h[r] - (l ? h[l - 1] * p[r - l + 1] % mod : 0));
-    return hash < 0 ? hash + mod : hash;
+      (h[r] - (l ? h[l - 1] * ppow[r - l + 1] % MOD : 0));
+    return hash < 0 ? hash + MOD : hash;
   }
-  ll query_inv(int l, int r) {
+
+  ll qry_inv(int l, int r) {
     ll hash =
       (hi[l] -
-       (r + 1 < n ? hi[r + 1] * p[r - l + 1] % mod : 0));
-    return hash < 0 ? hash + mod : hash;
+       (r + 1 < n ? hi[r + 1] * ppow[r - l + 1] % MOD : 0));
+    return hash < 0 ? hash + MOD : hash;
   }
 };