Exercism: RPN calculator

rust/rpn-calculator/.exercism/config.json

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+{
+  "blurb": "Use some of `Vec`'s methods to evaluate Reverse Polish notation",
+  "authors": [
+    "cwhakes"
+  ],
+  "files": {
+    "solution": [
+      "src/lib.rs"
+    ],
+    "test": [
+      "tests/rpn-calculator.rs"
+    ],
+    "exemplar": [
+      ".meta/exemplar.rs"
+    ]
+  }
+}

rust/rpn-calculator/.exercism/metadata.json

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+{"track":"rust","exercise":"rpn-calculator","id":"9044a3c6e14743789782a063e43a96b2","url":"https://exercism.org/tracks/rust/exercises/rpn-calculator","handle":"JBiason","is_requester":true,"auto_approve":false}

rust/rpn-calculator/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "rpn_calculator"
+version = "0.1.0"

rust/rpn-calculator/Cargo.toml

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+[package]
+name = "rpn_calculator"
+version = "0.1.0"
+edition = "2018"

rust/rpn-calculator/HELP.md

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+# Help
+
+## Running the tests
+
+Execute the tests with:
+
+```bash
+$ cargo test
+```
+
+All but the first test have been ignored. After you get the first test to
+pass, open the tests source file which is located in the `tests` directory
+and remove the `#[ignore]` flag from the next test and get the tests to pass
+again. Each separate test is a function with `#[test]` flag above it.
+Continue, until you pass every test.
+
+If you wish to run _only ignored_ tests without editing the tests source file, use:
+
+```bash
+$ cargo test -- --ignored
+```
+
+If you are using Rust 1.51 or later, you can run _all_ tests with
+
+```bash
+$ cargo test -- --include-ignored
+```
+
+To run a specific test, for example `some_test`, you can use:
+
+```bash
+$ cargo test some_test
+```
+
+If the specific test is ignored, use:
+
+```bash
+$ cargo test some_test -- --ignored
+```
+
+To learn more about Rust tests refer to the online [test documentation][rust-tests].
+
+[rust-tests]: https://doc.rust-lang.org/book/ch11-02-running-tests.html
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit src/lib.rs` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [Rust track's documentation](https://exercism.org/docs/tracks/rust)
+- [Exercism's support channel on gitter](https://gitter.im/exercism/support)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+## Rust Installation
+
+Refer to the [exercism help page][help-page] for Rust installation and learning
+resources.
+
+## Submitting the solution
+
+Generally you should submit all files in which you implemented your solution (`src/lib.rs` in most cases). If you are using any external crates, please consider submitting the `Cargo.toml` file. This will make the review process faster and clearer.
+
+## Feedback, Issues, Pull Requests
+
+The GitHub [track repository][github] is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!
+
+If you want to know more about Exercism, take a look at the [contribution guide].
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
+
+[help-page]: https://exercism.io/tracks/rust/learning
+[github]: https://github.com/exercism/rust
+[contribution guide]: https://exercism.io/docs/community/contributors

rust/rpn-calculator/HINTS.md

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+# Hints
+
+## General
+
+- Look at the documentation for `std::vec::Vec`'s [`push()`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.push) and ['pop()`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.pop) methods.

rust/rpn-calculator/README.md

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+# RPN Calculator
+
+Welcome to RPN Calculator on Exercism's Rust Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
+
+## Introduction
+
+[Stacks](https://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29) are a type of collection commonly used in computer science.
+They are defined by their two key operations: **push** and **pop**.
+**Push** adds an element to the top of the stack.
+**Pop** removes and returns the topmost element.
+
+Think of a stack like a stack of plates.
+You can either add a plate to the top of the stack or take the topmost plate.
+To access something further down, you have to remove all of the plates above it.
+
+Rust's vector implementation, [`std::vec::Vec`](https://doc.rust-lang.org/std/vec/struct.Vec.html), can be used as a stack by using its `push()` and `pop()` methods.
+Naturally, `push()` adds an element to the end of the `Vec` and `pop()` removes and returns the last element.
+These operation can be very fast (O(1) in [Big O Notation](https://en.wikipedia.org/wiki/Big_O_notation)),
+so they are one of the most idiomatic ways to use a `Vec`.
+
+Stacks are useful to hold arbitrary numbers of elements in a specific order.
+Because the last element inserted is the first element returned,
+stacks are commonly refered to as **LIFO** (Last-In, First-Out).
+This inherent ordering can be used for many things,
+including tracking state when evaulating **Reverse Polish notation**.
+
+## Instructions
+
+## 1. Overview
+
+[Reverse Polish notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation) (RPN) is a way of writing mathematical expressions.
+Unlike in traditional infix notation, RPN operators *follow* their operands.
+For example, instead of writing:
+
+```
+2 + 2
+```
+
+you would write:
+
+```
+2 2 +
+```
+
+The major benefit of Reverse Polish notation is that it is much simpler to parse than infix notation.
+RPN eliminates the need for order of operations or parentheses in complex expressions.
+For example:
+
+```
+(4 + 8) / (7 - 5)
+```
+
+can be written as
+
+```
+4 8 + 7 5 - /
+```
+
+In both cases, the expression evaluates to 6.
+
+## 2. Example
+
+Lets manually evaluate that complex expression.
+As we learned in the introduction, evaluation of RPN requires a stack.
+This stack is used to hold numeric values that the operators operate on.
+We start our calculator with an empty stack and then evaluate each element one at a time.
+
+First, we encounter a `4`,
+so we push it onto our freshly created stack.
+
+```
+4
+```
+
+Next, we encounter an `8`.
+We also push that onto the stack.
+
+```
+4 8
+```
+
+Now, we encounter a `+`.
+We pop off the two topmost values (4 and 8),
+add them together,
+and push the sum back onto the stack.
+
+```
+12
+```
+
+We do something similar for `7`, `5`, and `-`:
+
+```
+12 7
+12 7 5
+12 2
+```
+
+Now we encounter a `/`.
+Even though we last encountered a `-`,
+there are two elements on the stack.
+We pop off the two elements,
+divide them,
+and push the result back onto the stack.
+
+```
+6
+```
+
+Finally, since there is exactly one element on the stack,
+we can say the expression evaluated to 6.
+
+## 3. Goal
+
+Your goal is to write a calculator to evaluate a list of inputs ordered by Reverse Polish notation.
+You are given the following enum and stubbed function as a starting point.
+
+```rust
+#[derive(Debug)]
+pub enum CalculatorInput {
+    Add,
+    Subtract,
+    Multiply,
+    Divide,
+    Value(i32),
+}
+  
+pub fn evaluate(inputs: &[CalculatorInput]) -> Option<i32> {
+    unimplemented!(
+		"Given the inputs: {:?}, evaluate them as though they were a Reverse Polish notation expression",
+		inputs,
+	);
+}
+```
+
+## Source
+
+### Created by
+
+- @cwhakes

rust/rpn-calculator/src/lib.rs

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+#[derive(Debug)]
+pub enum CalculatorInput {
+    Add,
+    Subtract,
+    Multiply,
+    Divide,
+    Value(i32),
+}
+
+pub fn evaluate(inputs: &[CalculatorInput]) -> Option<i32> {
+    let mut stack: Vec<i32> = Vec::new();
+    let last_result = inputs.iter().fold(None, |_, entry| match entry {
+        CalculatorInput::Value(x) => {
+            stack.push(*x);
+            Some(*x)
+        }
+        CalculatorInput::Add => {
+            let val1 = stack.pop()?;
+            let val2 = stack.pop()?;
+            let result = val2 + val1;
+            stack.push(result);
+            Some(result)
+        }
+        CalculatorInput::Subtract => {
+            let val1 = stack.pop()?;
+            let val2 = stack.pop()?;
+            let result = val2 - val1;
+            stack.push(result);
+            Some(result)
+        }
+        CalculatorInput::Multiply => {
+            let val1 = stack.pop()?;
+            let val2 = stack.pop()?;
+            let result = val2 * val1;
+            stack.push(result);
+            Some(result)
+        }
+        CalculatorInput::Divide => {
+            let val1 = stack.pop()?;
+            let val2 = stack.pop()?;
+            let result = val2 / val1;
+            stack.push(result);
+            Some(result)
+        }
+    });
+
+    if stack.len() == 1 {
+        last_result
+    } else {
+        // too many operands
+        None
+    }
+}

rust/rpn-calculator/tests/rpn-calculator.rs

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+use rpn_calculator::*;
+
+fn calculator_input(s: &str) -> Vec<CalculatorInput> {
+    s.split_whitespace()
+        .map(|s| match s {
+            "+" => CalculatorInput::Add,
+            "-" => CalculatorInput::Subtract,
+            "*" => CalculatorInput::Multiply,
+            "/" => CalculatorInput::Divide,
+            n => CalculatorInput::Value(n.parse().unwrap()),
+        })
+        .collect()
+}
+
+#[test]
+fn test_empty_input_returns_none() {
+    let input = calculator_input("");
+    assert_eq!(evaluate(&input), None);
+}
+
+#[test]
+fn test_simple_value() {
+    let input = calculator_input("10");
+    assert_eq!(evaluate(&input), Some(10));
+}
+
+#[test]
+fn test_simple_addition() {
+    let input = calculator_input("2 2 +");
+    assert_eq!(evaluate(&input), Some(4));
+}
+
+#[test]
+fn test_simple_subtraction() {
+    let input = calculator_input("7 11 -");
+    assert_eq!(evaluate(&input), Some(-4));
+}
+
+#[test]
+fn test_simple_multiplication() {
+    let input = calculator_input("6 9 *");
+    assert_eq!(evaluate(&input), Some(54));
+}
+
+#[test]
+fn test_simple_division() {
+    let input = calculator_input("57 19 /");
+    assert_eq!(evaluate(&input), Some(3));
+}
+
+#[test]
+fn test_complex_operation() {
+    let input = calculator_input("4 8 + 7 5 - /");
+    assert_eq!(evaluate(&input), Some(6));
+}
+
+#[test]
+fn test_too_few_operands_returns_none() {
+    let input = calculator_input("2 +");
+    assert_eq!(evaluate(&input), None);
+}
+
+#[test]
+fn test_too_many_operands_returns_none() {
+    let input = calculator_input("2 2");
+    assert_eq!(evaluate(&input), None);
+}