feat: 20 - factorial digit sum

readme.md

@@ -17,7 +17,7 @@ Here is a [link to my profile](https://projecteuler.net/progress=newtykins).
 
 ## Challenge Completion
 
-### 23 out of 100 public challenges completed.
+### 25 out of 100 public challenges completed.
 
 - [x] 1 - [Multiples of 3 or 5](src/bin/1.rs)
 - [x] 2 - [Even Fibonacci numbers](src/bin/2.rs)
@@ -38,7 +38,7 @@ Here is a [link to my profile](https://projecteuler.net/progress=newtykins).
 - [x] 17 - [Number letter counts](src/bin/17.rs)
 - [x] 18 - [Maximum path sum I](src/bin/18.rs)
 - [x] 19 - Counting Sundays
-- [ ] 20 - Factorial digit sum
+- [x] 20 - Factorial digit sum
 - [ ] 21 - Amicable numbers
 - [x] 22 - [Names scores](src/bin/22.rs)
 - [ ] 23 - Non-abundant sums

src/bin/16.rs

@@ -5,11 +5,11 @@ Problem 16 - Power digit sum
 What is the sum of the digits of the number 2 1000 ?
 */
 
-fn power_digit_sum<const POWER: usize>(base: usize) -> usize {
-    let mut digits = [0u8; POWER];
+fn power_digit_sum(base: usize, power: usize) -> usize {
+    let mut digits = vec![0u8; power];
     digits[0] = 1; // 2^0 = 1
 
-    for _ in 0..POWER {
+    for _ in 0..power {
         let mut carry = 0;
 
         for digit in digits.iter_mut() {
@@ -23,7 +23,7 @@ fn power_digit_sum<const POWER: usize>(base: usize) -> usize {
 }
 
 pub fn main() {
-    let sum = power_digit_sum::<1000>(2);
+    let sum = power_digit_sum(2, 1000);
 
     println!("The sum of the digits of the number 2^1000 is {}!", sum);
 }

src/bin/20.rs

@@ -0,0 +1,34 @@
+/*
+Problem 20 - Factorial Digit Sum
+
+$n!$ means $n \times (n - 1) \times \cdots \times 3 \times 2 \times 1$.
+For example, $10! = 10 \times 9 \times \cdots \times 3 \times 2 \times 1 = 3628800$, and the sum of the digits in the number $10!$ is $3 + 6 + 2 + 8 + 8 + 0 + 0 = 27$.
+Find the sum of the digits in the number $100!$.
+*/
+
+fn multiply(digits: &mut Vec<u8>, multiplier: usize) {
+    let mut carry = 0usize;
+
+    for digit in digits.iter_mut() {
+        let product = *digit as usize * multiplier + carry;
+        *digit = (product % 10) as u8;
+        carry = product / 10;
+    }
+
+    while carry > 0 {
+        digits.push((carry % 10) as u8);
+        carry /= 10;
+    }
+}
+
+pub fn main() {
+    let mut digits = vec![1];
+
+    for num in 2..=100 {
+        multiply(&mut digits, num);
+    }
+
+    let sum: usize = digits.iter().map(|&d| d as usize).sum();
+
+    print!("The sum of the digits in the number 100! is {}!", sum);
+}

src/commands/new.rs

@@ -17,40 +17,42 @@ pub async fn execute(number: Option<u8>) -> Result<()> {
     file.write(problem.file_body().as_bytes())?;
     drop(file);
 
-    // open readme
-    let mut readme_file = OpenOptions::new().read(true).open(README.clone())?;
+    if problem.number <= 100 {
+        // open readme
+        let mut readme_file = OpenOptions::new().read(true).open(README.clone())?;
 
-    let readme_content = {
-        // read
-        let mut content = BufReader::new(&readme_file)
-            .lines()
-            .map(|s| s.unwrap())
-            .collect::<Vec<String>>()
-            .join("\n");
+        let readme_content = {
+            // read
+            let mut content = BufReader::new(&readme_file)
+                .lines()
+                .map(|s| s.unwrap())
+                .collect::<Vec<String>>()
+                .join("\n");
 
+            drop(readme_file);
+
+            // mark problem as done on readme
+            content = content.replace(
+                &format!("\n- [ ] {}", problem.number),
+                &format!("\n- [x] {}", problem.number),
+            );
+
+            // update completed count
+            let completion_regex = Regex::new(r"(\d+) out of")?;
+            let problem_count = fs::read_dir(SOLUTIONS.clone())?.count();
+
+            content = completion_regex
+                .replace(&content, format!("{} out of", problem_count))
+                .to_string();
+
+            content
+        };
+
+        // write the new content to the readme
+        readme_file = OpenOptions::new().write(true).open(README.clone())?;
+        readme_file.write(readme_content.as_bytes())?;
         drop(readme_file);
-
-        // mark problem as done on readme
-        content = content.replace(
-            &format!("\n- [ ] {}", problem.number),
-            &format!("\n- [x] {}", problem.number),
-        );
-
-        // update completed count
-        let completion_regex = Regex::new(r"(\d+) out of")?;
-        let problem_count = fs::read_dir(SOLUTIONS.clone())?.count();
-
-        content = completion_regex
-            .replace(&content, format!("{} out of", problem_count))
-            .to_string();
-
-        content
-    };
-
-    // write the new content to the readme
-    readme_file = OpenOptions::new().write(true).open(README.clone())?;
-    readme_file.write(readme_content.as_bytes())?;
-    drop(readme_file);
+    }
 
     // add the problem to the run command
     let mut run_file = OpenOptions::new()

src/commands/run.rs

@@ -42,6 +42,8 @@ mod thirteen;
 mod three;
 #[path = "../bin/12.rs"]
 mod twelve;
+#[path = "../bin/20.rs"]
+mod twenty;
 #[path = "../bin/27.rs"]
 mod twenty_seven;
 #[path = "../bin/22.rs"]
@@ -87,6 +89,7 @@ pub async fn execute(
         27 => twenty_seven::main(),
         67 => sixty_seven::main(),
         19 => nineteen::main(),
+        20 => twenty::main(),
         _ => {
             exists = false;
             println!(

src/problem.rs

@@ -21,8 +21,6 @@ impl Problem {
                     // ensure the number is in bound
                     if n <= 0 {
                         Err("Please enter a number greater than 0".to_string())
-                    } else if n > 100 {
-                        Err("Please enter a number less than or equal to 100".to_string())
                     }
                     // ensure that the problem has not already got a file associated with it
                     else {
@@ -64,7 +62,7 @@ impl Problem {
         let number = number
             .unwrap_or_else(|| Self::prompt_number("Please select a problem:", false).unwrap());
 
-        let body = reqwest::get(format!("https://projecteuler.net/minimal={number}"))
+        let body = reqwest::get(format!("https://projecteuler.net/problem={number}"))
             .await?
             .text()
             .await?;