port over the nth command from nushell (#454)

* port over the nth command from nushell

* remove a line of redundant code

* must sort the rows or else if the rows are not from low to high this crashes engine-q

crates/nu-command/src/default_context.rs

@@ -113,6 +113,7 @@ pub fn create_default_context() -> EngineState {
             Mkdir,
             Module,
             Mv,
+            Nth,
             ParEach,
             Parse,
             Prepend,

crates/nu-command/src/filters/mod.rs

@@ -9,6 +9,7 @@ mod get;
 mod last;
 mod length;
 mod lines;
+mod nth;
 mod par_each;
 mod prepend;
 mod range;
@@ -34,6 +35,7 @@ pub use get::Get;
 pub use last::Last;
 pub use length::Length;
 pub use lines::Lines;
+pub use nth::Nth;
 pub use par_each::ParEach;
 pub use prepend::Prepend;
 pub use range::Range;

crates/nu-command/src/filters/nth.rs

@@ -0,0 +1,152 @@
+use nu_engine::CallExt;
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, IntoInterruptiblePipelineData, PipelineData, PipelineIterator, ShellError,
+    Signature, Span, SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct Nth;
+
+impl Command for Nth {
+    fn name(&self) -> &str {
+        "nth"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("nth")
+            .rest("rest", SyntaxShape::Int, "the number of the row to return")
+            .switch("skip", "Skip the rows instead of selecting them", Some('s'))
+            .category(Category::Filters)
+    }
+
+    fn usage(&self) -> &str {
+        "Return or skip only the selected rows."
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                example: "[sam,sarah,2,3,4,5] | nth 0 1 2",
+                description: "Get the first, second, and third row",
+                result: Some(Value::List {
+                    vals: vec![
+                        Value::test_string("sam"),
+                        Value::test_string("sarah"),
+                        Value::test_int(2),
+                    ],
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                example: "[0,1,2,3,4,5] | nth 0 1 2",
+                description: "Get the first, second, and third row",
+                result: Some(Value::List {
+                    vals: vec![Value::test_int(0), Value::test_int(1), Value::test_int(2)],
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                example: "[0,1,2,3,4,5] | nth -s 0 1 2",
+                description: "Skip the first, second, and third row",
+                result: Some(Value::List {
+                    vals: vec![Value::test_int(3), Value::test_int(4), Value::test_int(5)],
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                example: "[0,1,2,3,4,5] | nth 0 2 4",
+                description: "Get the first, third, and fifth row",
+                result: Some(Value::List {
+                    vals: vec![Value::test_int(0), Value::test_int(2), Value::test_int(4)],
+                    span: Span::unknown(),
+                }),
+            },
+            Example {
+                example: "[0,1,2,3,4,5] | nth 2 0 4",
+                description: "Get the first, third, and fifth row",
+                result: Some(Value::List {
+                    vals: vec![Value::test_int(0), Value::test_int(2), Value::test_int(4)],
+                    span: Span::unknown(),
+                }),
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let mut rows: Vec<usize> = call.rest(engine_state, stack, 0)?;
+        rows.sort_unstable();
+        let skip = call.has_flag("skip");
+        let pipeline_iter: PipelineIterator = input.into_iter();
+
+        Ok(NthIterator {
+            input: pipeline_iter,
+            rows,
+            skip,
+            current: 0,
+        }
+        .into_pipeline_data(engine_state.ctrlc.clone()))
+    }
+}
+
+struct NthIterator {
+    input: PipelineIterator,
+    rows: Vec<usize>,
+    skip: bool,
+    current: usize,
+}
+
+impl Iterator for NthIterator {
+    type Item = Value;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if !self.skip {
+                if let Some(row) = self.rows.get(0) {
+                    if self.current == *row {
+                        self.rows.remove(0);
+                        self.current += 1;
+                        return self.input.next();
+                    } else {
+                        self.current += 1;
+                        let _ = self.input.next();
+                        continue;
+                    }
+                } else {
+                    return None;
+                }
+            } else if let Some(row) = self.rows.get(0) {
+                if self.current == *row {
+                    self.rows.remove(0);
+                    self.current += 1;
+                    let _ = self.input.next();
+                    continue;
+                } else {
+                    self.current += 1;
+                    return self.input.next();
+                }
+            } else {
+                return self.input.next();
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Nth {})
+    }
+}