nu_command/filters/
last.rs

1use nu_engine::command_prelude::*;
2use nu_protocol::shell_error::io::IoError;
3use std::{collections::VecDeque, io::Read};
4
5#[cfg(feature = "sqlite")]
6use crate::database::SQLiteQueryBuilder;
7
8#[derive(Clone)]
9pub struct Last;
10
11impl Command for Last {
12    fn name(&self) -> &str {
13        "last"
14    }
15
16    fn signature(&self) -> Signature {
17        Signature::build("last")
18            .input_output_types(vec![
19                (
20                    // TODO: This is too permissive; if we could express this
21                    // using a type parameter it would be List<T> -> T.
22                    Type::List(Box::new(Type::Any)),
23                    Type::Any,
24                ),
25                (Type::Binary, Type::Binary),
26                (Type::Range, Type::Any),
27            ])
28            .optional(
29                "rows",
30                SyntaxShape::Int,
31                "Starting from the back, the number of rows to return.",
32            )
33            .allow_variants_without_examples(true)
34            .switch("strict", "Throw an error if input is empty.", Some('s'))
35            .category(Category::Filters)
36    }
37
38    fn search_terms(&self) -> Vec<&str> {
39        vec!["tail", "end"]
40    }
41
42    fn description(&self) -> &str {
43        "Return only the last several rows of the input. Counterpart of `first`. Opposite of `drop`."
44    }
45
46    fn examples(&self) -> Vec<Example<'_>> {
47        vec![
48            Example {
49                example: "[1,2,3] | last 2",
50                description: "Return the last 2 items of a list/table.",
51                result: Some(Value::list(
52                    vec![Value::test_int(2), Value::test_int(3)],
53                    Span::test_data(),
54                )),
55            },
56            Example {
57                example: "[1,2,3] | last",
58                description: "Return the last item of a list/table.",
59                result: Some(Value::test_int(3)),
60            },
61            Example {
62                example: "0x[01 23 45] | last 2",
63                description: "Return the last 2 bytes of a binary value.",
64                result: Some(Value::binary(vec![0x23, 0x45], Span::test_data())),
65            },
66            Example {
67                example: "1..3 | last",
68                description: "Return the last item of a range.",
69                result: Some(Value::test_int(3)),
70            },
71        ]
72    }
73
74    fn run(
75        &self,
76        engine_state: &EngineState,
77        stack: &mut Stack,
78        call: &Call,
79        input: PipelineData,
80    ) -> Result<PipelineData, ShellError> {
81        let head = call.head;
82        let rows = call.opt::<usize>(engine_state, stack, 0)?;
83        let strict_mode = call.has_flag(engine_state, stack, "strict")?;
84
85        // FIXME: Please read the FIXME message in `first.rs`'s `first_helper` implementation.
86        // It has the same issue.
87        let return_single_element = rows.is_none();
88        let rows = rows.unwrap_or(1);
89
90        let mut input = input;
91        let metadata = input.take_metadata();
92
93        // Count is 0: return empty data immediately.
94        //
95        // The main `match` below is not safe for this case-`last` reads binary streams in chunks
96        // and sqlite paths may still execute. For "take nothing" we only produce an empty value:
97        // empty binary (and clear pipeline `content_type` for binary) or an empty list, with other
98        // metadata unchanged.
99        if rows == 0 {
100            return match input {
101                PipelineData::Value(val, _) if matches!(&val, Value::Binary { .. }) => Ok(
102                    Value::binary(Vec::new(), val.span()).into_pipeline_data_with_metadata(
103                        metadata.map(|m| m.with_content_type(None)),
104                    ),
105                ),
106                PipelineData::ByteStream(stream, _) => {
107                    if stream.type_().is_binary_coercible() {
108                        let span = stream.span();
109                        Ok(
110                            Value::binary(Vec::new(), span).into_pipeline_data_with_metadata(
111                                metadata.map(|m| m.with_content_type(None)),
112                            ),
113                        )
114                    } else {
115                        Ok(
116                            Value::list(Vec::new(), head)
117                                .into_pipeline_data_with_metadata(metadata),
118                        )
119                    }
120                }
121                _ => Ok(Value::list(Vec::new(), head).into_pipeline_data_with_metadata(metadata)),
122            };
123        }
124
125        match input {
126            PipelineData::ListStream(_, _) | PipelineData::Value(Value::Range { .. }, _) => {
127                let iterator = input.into_iter_strict(head)?;
128
129                // only keep the last `rows` in memory
130                let mut buf = VecDeque::new();
131
132                for row in iterator {
133                    engine_state.signals().check(&head)?;
134                    if buf.len() == rows {
135                        buf.pop_front();
136                    }
137                    buf.push_back(row);
138                }
139
140                if return_single_element {
141                    if let Some(last) = buf.pop_back() {
142                        Ok(last.into_pipeline_data_with_metadata(metadata))
143                    } else if strict_mode {
144                        Err(ShellError::AccessEmptyContent { span: head })
145                    } else {
146                        // There are no values, so return nothing instead of an error so
147                        // that users can pipe this through 'default' if they want to.
148                        Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
149                    }
150                } else {
151                    Ok(Value::list(buf.into(), head).into_pipeline_data_with_metadata(metadata))
152                }
153            }
154            PipelineData::Value(val, _) => {
155                let span = val.span();
156                match val {
157                    Value::List { mut vals, .. } => {
158                        if return_single_element {
159                            if let Some(v) = vals.pop() {
160                                Ok(v.into_pipeline_data_with_metadata(metadata))
161                            } else if strict_mode {
162                                Err(ShellError::AccessEmptyContent { span: head })
163                            } else {
164                                // There are no values, so return nothing instead of an error so
165                                // that users can pipe this through 'default' if they want to.
166                                Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
167                            }
168                        } else {
169                            let i = vals.len().saturating_sub(rows);
170                            vals.drain(..i);
171                            Ok(Value::list(vals, span).into_pipeline_data_with_metadata(metadata))
172                        }
173                    }
174                    Value::Binary { mut val, .. } => {
175                        let binary_meta = metadata.map(|m| m.with_content_type(None));
176                        if return_single_element {
177                            if let Some(val) = val.pop() {
178                                Ok(Value::int(val.into(), span)
179                                    .into_pipeline_data_with_metadata(binary_meta))
180                            } else if strict_mode {
181                                Err(ShellError::AccessEmptyContent { span: head })
182                            } else {
183                                // There are no values, so return nothing instead of an error so
184                                // that users can pipe this through 'default' if they want to.
185                                Ok(Value::nothing(head)
186                                    .into_pipeline_data_with_metadata(binary_meta))
187                            }
188                        } else {
189                            let i = val.len().saturating_sub(rows);
190                            val.drain(..i);
191                            Ok(Value::binary(val, span)
192                                .into_pipeline_data_with_metadata(binary_meta))
193                        }
194                    }
195                    // Propagate errors by explicitly matching them before the final case.
196                    Value::Error { error, .. } => Err(*error),
197                    #[cfg(feature = "sqlite")]
198                    // Pushdown optimization: handle 'last' on SQLiteQueryBuilder for lazy SQL execution
199                    Value::Custom {
200                        val: custom_val,
201                        internal_span,
202                        ..
203                    } => {
204                        if let Some(table) =
205                            custom_val.as_any().downcast_ref::<SQLiteQueryBuilder>()
206                        {
207                            if return_single_element {
208                                // For single element, ORDER BY rowid DESC LIMIT 1
209                                let new_table = table
210                                    .clone()
211                                    .with_order_by("rowid DESC".to_string())
212                                    .with_limit(1);
213                                let result = new_table.execute(head)?;
214                                let value = result.into_value(head)?;
215                                if let Value::List { vals, .. } = value {
216                                    if let Some(val) = vals.into_iter().next() {
217                                        Ok(val.into_pipeline_data_with_metadata(metadata))
218                                    } else if strict_mode {
219                                        Err(ShellError::AccessEmptyContent { span: head })
220                                    } else {
221                                        // There are no values, so return nothing instead of an error so
222                                        // that users can pipe this through 'default' if they want to.
223                                        Ok(Value::nothing(head)
224                                            .into_pipeline_data_with_metadata(metadata))
225                                    }
226                                } else {
227                                    Err(ShellError::NushellFailed {
228                                        msg: "Expected list from SQLiteQueryBuilder".into(),
229                                    })
230                                }
231                            } else {
232                                // For multiple, ORDER BY rowid DESC LIMIT rows
233                                let new_table = table
234                                    .clone()
235                                    .with_order_by("rowid DESC".to_string())
236                                    .with_limit(rows as i64);
237                                let result = new_table.execute(head)?;
238                                let value = result.into_value(head)?;
239
240                                if let Value::List { mut vals, .. } = value {
241                                    // Reverse the results to restore original order
242                                    vals.reverse();
243                                    Ok(Value::list(vals, head)
244                                        .into_pipeline_data_with_metadata(metadata))
245                                } else {
246                                    Ok(value.into_pipeline_data_with_metadata(metadata))
247                                }
248                            }
249                        } else {
250                            Err(ShellError::OnlySupportsThisInputType {
251                                exp_input_type: "list, binary or range".into(),
252                                wrong_type: custom_val.type_name(),
253                                dst_span: head,
254                                src_span: internal_span,
255                            })
256                        }
257                    }
258                    other => Err(ShellError::OnlySupportsThisInputType {
259                        exp_input_type: "list, binary or range".into(),
260                        wrong_type: other.get_type().to_string(),
261                        dst_span: head,
262                        src_span: other.span(),
263                    }),
264                }
265            }
266            PipelineData::ByteStream(stream, ..) => {
267                if stream.type_().is_binary_coercible() {
268                    let span = stream.span();
269                    let byte_meta = metadata.map(|m| m.with_content_type(None));
270                    if let Some(mut reader) = stream.reader() {
271                        // Have to be a bit tricky here, but just consume into a VecDeque that we
272                        // shrink to fit each time
273                        const TAKE: u64 = 8192;
274                        let mut buf = VecDeque::with_capacity(rows + TAKE as usize);
275                        loop {
276                            let taken = std::io::copy(&mut (&mut reader).take(TAKE), &mut buf)
277                                .map_err(|err| IoError::new(err, span, None))?;
278                            if buf.len() > rows {
279                                buf.drain(..(buf.len() - rows));
280                            }
281                            if taken < TAKE {
282                                // This must be EOF.
283                                if return_single_element {
284                                    if !buf.is_empty() {
285                                        return Ok(Value::int(buf[0] as i64, head)
286                                            .into_pipeline_data_with_metadata(byte_meta));
287                                    } else if strict_mode {
288                                        return Err(ShellError::AccessEmptyContent { span: head });
289                                    } else {
290                                        // There are no values, so return nothing instead of an error so
291                                        // that users can pipe this through 'default' if they want to.
292                                        return Ok(Value::nothing(head)
293                                            .into_pipeline_data_with_metadata(byte_meta));
294                                    }
295                                } else {
296                                    return Ok(Value::binary(buf, head)
297                                        .into_pipeline_data_with_metadata(byte_meta));
298                                }
299                            }
300                        }
301                    } else {
302                        Ok(Value::nothing(head).into_pipeline_data_with_metadata(byte_meta))
303                    }
304                } else {
305                    Err(ShellError::OnlySupportsThisInputType {
306                        exp_input_type: "list, binary or range".into(),
307                        wrong_type: stream.type_().describe().into(),
308                        dst_span: head,
309                        src_span: stream.span(),
310                    })
311                }
312            }
313            PipelineData::Empty => Err(ShellError::OnlySupportsThisInputType {
314                exp_input_type: "list, binary or range".into(),
315                wrong_type: "null".into(),
316                dst_span: call.head,
317                src_span: call.head,
318            }),
319        }
320    }
321}
322
323#[cfg(test)]
324mod test {
325    use super::*;
326
327    #[test]
328    fn test_examples() -> nu_test_support::Result {
329        nu_test_support::test().examples(Last)
330    }
331}
nu_command/filters/last.rs

nu_command/filters/
last.rs
