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use nu_engine::command_prelude::*;
use nu_protocol::shell_error::io::IoError;
use std::{collections::VecDeque, io::Read};
#[derive(Clone)]
pub struct Last;
impl Command for Last {
fn name(&self) -> &str {
"last"
}
fn signature(&self) -> Signature {
Signature::build("last")
.input_output_types(vec![
(
// TODO: This is too permissive; if we could express this
// using a type parameter it would be List<T> -> T.
Type::List(Box::new(Type::Any)),
Type::Any,
),
(Type::Binary, Type::Binary),
(Type::Range, Type::Any),
])
.optional(
"rows",
SyntaxShape::Int,
"Starting from the back, the number of rows to return.",
)
.category(Category::Filters)
}
fn description(&self) -> &str {
"Return only the last several rows of the input. Counterpart of `first`. Opposite of `drop`."
}
fn examples(&self) -> Vec<Example> {
vec![
Example {
example: "[1,2,3] | last 2",
description: "Return the last 2 items of a list/table",
result: Some(Value::list(
vec![Value::test_int(2), Value::test_int(3)],
Span::test_data(),
)),
},
Example {
example: "[1,2,3] | last",
description: "Return the last item of a list/table",
result: Some(Value::test_int(3)),
},
Example {
example: "0x[01 23 45] | last 2",
description: "Return the last 2 bytes of a binary value",
result: Some(Value::binary(vec![0x23, 0x45], Span::test_data())),
},
Example {
example: "1..3 | last",
description: "Return the last item of a range",
result: Some(Value::test_int(3)),
},
]
}
fn run(
&self,
engine_state: &EngineState,
stack: &mut Stack,
call: &Call,
input: PipelineData,
) -> Result<PipelineData, ShellError> {
let head = call.head;
let rows: Option<Spanned<i64>> = call.opt(engine_state, stack, 0)?;
// FIXME: Please read the FIXME message in `first.rs`'s `first_helper` implementation.
// It has the same issue.
let return_single_element = rows.is_none();
let rows = if let Some(rows) = rows {
if rows.item < 0 {
return Err(ShellError::NeedsPositiveValue { span: rows.span });
} else {
rows.item as usize
}
} else {
1
};
let metadata = input.metadata();
// early exit for `last 0`
if rows == 0 {
return Ok(Value::list(Vec::new(), head).into_pipeline_data_with_metadata(metadata));
}
match input {
PipelineData::ListStream(_, _) | PipelineData::Value(Value::Range { .. }, _) => {
let iterator = input.into_iter_strict(head)?;
// only keep the last `rows` in memory
let mut buf = VecDeque::new();
for row in iterator {
engine_state.signals().check(&head)?;
if buf.len() == rows {
buf.pop_front();
}
buf.push_back(row);
}
if return_single_element {
if let Some(last) = buf.pop_back() {
Ok(last.into_pipeline_data())
} else {
Err(ShellError::AccessEmptyContent { span: head })
}
} else {
Ok(Value::list(buf.into(), head).into_pipeline_data_with_metadata(metadata))
}
}
PipelineData::Value(val, _) => {
let span = val.span();
match val {
Value::List { mut vals, .. } => {
if return_single_element {
if let Some(v) = vals.pop() {
Ok(v.into_pipeline_data())
} else {
Err(ShellError::AccessEmptyContent { span: head })
}
} else {
let i = vals.len().saturating_sub(rows);
vals.drain(..i);
Ok(Value::list(vals, span).into_pipeline_data_with_metadata(metadata))
}
}
Value::Binary { mut val, .. } => {
if return_single_element {
if let Some(val) = val.pop() {
Ok(Value::int(val.into(), span).into_pipeline_data())
} else {
Err(ShellError::AccessEmptyContent { span: head })
}
} else {
let i = val.len().saturating_sub(rows);
val.drain(..i);
Ok(Value::binary(val, span).into_pipeline_data())
}
}
// Propagate errors by explicitly matching them before the final case.
Value::Error { error, .. } => Err(*error),
other => Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: other.get_type().to_string(),
dst_span: head,
src_span: other.span(),
}),
}
}
PipelineData::ByteStream(stream, ..) => {
if stream.type_().is_binary_coercible() {
let span = stream.span();
if let Some(mut reader) = stream.reader() {
// Have to be a bit tricky here, but just consume into a VecDeque that we
// shrink to fit each time
const TAKE: u64 = 8192;
let mut buf = VecDeque::with_capacity(rows + TAKE as usize);
loop {
let taken = std::io::copy(&mut (&mut reader).take(TAKE), &mut buf)
.map_err(|err| IoError::new(err, span, None))?;
if buf.len() > rows {
buf.drain(..(buf.len() - rows));
}
if taken < TAKE {
// This must be EOF.
if return_single_element {
if !buf.is_empty() {
return Ok(
Value::int(buf[0] as i64, head).into_pipeline_data()
);
} else {
return Err(ShellError::AccessEmptyContent { span: head });
}
} else {
return Ok(Value::binary(buf, head).into_pipeline_data());
}
}
}
} else {
Ok(PipelineData::Empty)
}
} else {
Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: stream.type_().describe().into(),
dst_span: head,
src_span: stream.span(),
})
}
}
PipelineData::Empty => Err(ShellError::OnlySupportsThisInputType {
exp_input_type: "list, binary or range".into(),
wrong_type: "null".into(),
dst_span: call.head,
src_span: call.head,
}),
}
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_examples() {
use crate::test_examples;
test_examples(Last {})
}
}