feat(euler): challenge 25 - 1000 digit fibonacci index

challenges/euler/readme.md

@@ -36,7 +36,7 @@ The source code can be found in the [src](src) directory. My thoughts about some
 -   [x] [22 - Names scores](src/22%20-%20Names%20scores.ts)
 -   [ ] 23 - Non-abundant sums
 -   [ ] 24 - Lexicographic permutations
--   [ ] 25 - 1000-digit Fibonacci number
+-   [x] [25 - 1000-digit Fibonacci number](src/25%20-%201000-digit%20Fibonacci%20number.ts)
 -   [ ] 26 - Reciprocal cycles
 -   [ ] 27 - Quadratic primes
 -   [ ] 28 - Number spiral diagonals

challenges/euler/src/25 - 1000-digit Fibonacci number.ts

@@ -0,0 +1,43 @@
+// The Fibonacci sequence is defined by the recurrence relation:
+// Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
+// The 12th term, F12, is the first term to contain three digits.
+// What is the index of the first term in the Fibonacci sequence to contain 1000 digits?
+export = {};
+
+function* fibonacciSequence(): Generator<bigint, bigint, bigint> {
+    let current = BigInt(1);
+    let a = BigInt(1);
+    let b = BigInt(1);
+
+    yield BigInt(1);
+
+    while (true) {
+        current = b;
+        yield current;
+
+        b = a + b;
+        a = current;
+    }
+}
+
+const firstTermWithXDigits = (sequence: Generator<bigint, bigint, bigint>, digitCount: number) => {
+    let { value: term } = sequence.next();
+    let index = 1;
+
+    while (term.toString().length !== digitCount) {
+        term = sequence.next().value;
+        index += 1;
+    }
+
+    return {
+        value: term,
+        index
+    };
+};
+
+// Output
+
+const fibonacci = fibonacciSequence();
+const firstTerm = firstTermWithXDigits(fibonacci, 1000);
+
+console.log(firstTerm);