nushell/crates/nu-command/src/filters/last.rs

use nu_engine::CallExt;

use nu_protocol::ast::Call;
use nu_protocol::engine::{Command, EngineState, Stack};
use nu_protocol::{PipelineData, ShellError, Signature, SyntaxShape, Value};

//use nu_protocol::{IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Value};
//use std::convert::TryInto;

#[derive(Clone)]
pub struct Last;

impl Command for Last {
    fn name(&self) -> &str {
        "last"
    }

    fn signature(&self) -> Signature {
        Signature::build("last").optional(
            "rows",
            SyntaxShape::Int,
            "starting from the back, the number of rows to return",
        )
    }

    fn usage(&self) -> &str {
        "Show only the last number of rows."
    }

    fn run(
        &self,
        engine_state: &EngineState,
        stack: &mut Stack,
        call: &Call,
        input: PipelineData,
    ) -> Result<PipelineData, ShellError> {
        let rows: Option<i64> = call.opt(engine_state, stack, 0)?;

        let iter_count: i64 = input.into_iter().count() as i64;

        let beginning_rows_to_skip = rows_to_skip(iter_count, rows);

        dbg!(beginning_rows_to_skip);

        // This code works fine and does the correct
        // calculation of the beginning_rows_to_skip
        // but it can not currently be used because
        // I am not able to clone the input

        /*
                let vlength = length(input)?;
                dbg!(vlength);

                let end_rows_desired = if let Some(quantity) = rows {
                    quantity
                } else {
                    1
                };

                let beginning_rows_to_skip = if end_rows_desired < vlength {
                    vlength - end_rows_desired
                } else {
                    0
                };


        */

        // This code works fine if I am able to get this value
        // So for now I am hard coding this number

        // The above code successfully calculates this number
        // and it all works if I was able to clone the input

        // it seems being able to
        // clone the input is important

        // As we were able to do that prior to your changes...

        /*
                let beginning_rows_to_skip = 2;

                match input {
                    PipelineData::Stream(stream) => {
                        dbg!("Stream");
                        Ok(stream
                            .skip(beginning_rows_to_skip.try_into().unwrap())
                            .into_pipeline_data())
                    }
                    PipelineData::Value(Value::List { vals, .. }) => {
                        dbg!("Value");
                        Ok(vals
                            .into_iter()
                            .skip(beginning_rows_to_skip.try_into().unwrap())
                            .into_pipeline_data())
                    }
                    _ => {
                        dbg!("Fall to the bottom");
                        Ok(PipelineData::Value(Value::Nothing { span: call.head }))
                    }
                }
        */
        Ok(PipelineData::Value(Value::Nothing { span: call.head }))
    }
}

fn rows_to_skip(count: i64, rows: Option<i64>) -> i64 {
    let end_rows_desired = if let Some(quantity) = rows {
        quantity
    } else {
        1
    };

    if end_rows_desired < count {
        return count - end_rows_desired;
    } else {
        return 0;
    };
}