shannon_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: Option<Spanned<i64>> = call.opt(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 = if let Some(rows) = rows {
89            if rows.item < 0 {
90                return Err(ShellError::NeedsPositiveValue { span: rows.span });
91            } else {
92                rows.item as usize
93            }
94        } else {
95            1
96        };
97
98        let metadata = input.metadata();
99
100        // early exit for `last 0`
101        if rows == 0 {
102            return Ok(Value::list(Vec::new(), head).into_pipeline_data_with_metadata(metadata));
103        }
104
105        match input {
106            PipelineData::ListStream(_, _) | PipelineData::Value(Value::Range { .. }, _) => {
107                let iterator = input.into_iter_strict(head)?;
108
109                // only keep the last `rows` in memory
110                let mut buf = VecDeque::new();
111
112                for row in iterator {
113                    engine_state.signals().check(&head)?;
114                    if buf.len() == rows {
115                        buf.pop_front();
116                    }
117                    buf.push_back(row);
118                }
119
120                if return_single_element {
121                    if let Some(last) = buf.pop_back() {
122                        Ok(last.into_pipeline_data())
123                    } else if strict_mode {
124                        Err(ShellError::AccessEmptyContent { span: head })
125                    } else {
126                        // There are no values, so return nothing instead of an error so
127                        // that users can pipe this through 'default' if they want to.
128                        Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
129                    }
130                } else {
131                    Ok(Value::list(buf.into(), head).into_pipeline_data_with_metadata(metadata))
132                }
133            }
134            PipelineData::Value(val, _) => {
135                let span = val.span();
136                match val {
137                    Value::List { mut vals, .. } => {
138                        if return_single_element {
139                            if let Some(v) = vals.pop() {
140                                Ok(v.into_pipeline_data())
141                            } else if strict_mode {
142                                Err(ShellError::AccessEmptyContent { span: head })
143                            } else {
144                                // There are no values, so return nothing instead of an error so
145                                // that users can pipe this through 'default' if they want to.
146                                Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
147                            }
148                        } else {
149                            let i = vals.len().saturating_sub(rows);
150                            vals.drain(..i);
151                            Ok(Value::list(vals, span).into_pipeline_data_with_metadata(metadata))
152                        }
153                    }
154                    Value::Binary { mut val, .. } => {
155                        if return_single_element {
156                            if let Some(val) = val.pop() {
157                                Ok(Value::int(val.into(), span).into_pipeline_data())
158                            } else if strict_mode {
159                                Err(ShellError::AccessEmptyContent { span: head })
160                            } else {
161                                // There are no values, so return nothing instead of an error so
162                                // that users can pipe this through 'default' if they want to.
163                                Ok(Value::nothing(head).into_pipeline_data_with_metadata(metadata))
164                            }
165                        } else {
166                            let i = val.len().saturating_sub(rows);
167                            val.drain(..i);
168                            Ok(Value::binary(val, span).into_pipeline_data())
169                        }
170                    }
171                    // Propagate errors by explicitly matching them before the final case.
172                    Value::Error { error, .. } => Err(*error),
173                    #[cfg(feature = "sqlite")]
174                    // Pushdown optimization: handle 'last' on SQLiteQueryBuilder for lazy SQL execution
175                    Value::Custom {
176                        val: custom_val,
177                        internal_span,
178                        ..
179                    } => {
180                        if let Some(table) =
181                            custom_val.as_any().downcast_ref::<SQLiteQueryBuilder>()
182                        {
183                            if return_single_element {
184                                // For single element, ORDER BY rowid DESC LIMIT 1
185                                let new_table = table
186                                    .clone()
187                                    .with_order_by("rowid DESC".to_string())
188                                    .with_limit(1);
189                                let result = new_table.execute(head)?;
190                                let value = result.into_value(head)?;
191                                if let Value::List { vals, .. } = value {
192                                    if let Some(val) = vals.into_iter().next() {
193                                        Ok(val.into_pipeline_data())
194                                    } else if strict_mode {
195                                        Err(ShellError::AccessEmptyContent { span: head })
196                                    } else {
197                                        // There are no values, so return nothing instead of an error so
198                                        // that users can pipe this through 'default' if they want to.
199                                        Ok(Value::nothing(head)
200                                            .into_pipeline_data_with_metadata(metadata))
201                                    }
202                                } else {
203                                    Err(ShellError::NushellFailed {
204                                        msg: "Expected list from SQLiteQueryBuilder".into(),
205                                    })
206                                }
207                            } else {
208                                // For multiple, ORDER BY rowid DESC LIMIT rows
209                                let new_table = table
210                                    .clone()
211                                    .with_order_by("rowid DESC".to_string())
212                                    .with_limit(rows as i64);
213                                let result = new_table.execute(head)?;
214                                let value = result.into_value(head)?;
215
216                                if let Value::List { mut vals, .. } = value {
217                                    // Reverse the results to restore original order
218                                    vals.reverse();
219                                    Ok(Value::list(vals, head)
220                                        .into_pipeline_data_with_metadata(metadata))
221                                } else {
222                                    Ok(value.into_pipeline_data_with_metadata(metadata))
223                                }
224                            }
225                        } else {
226                            Err(ShellError::OnlySupportsThisInputType {
227                                exp_input_type: "list, binary or range".into(),
228                                wrong_type: custom_val.type_name(),
229                                dst_span: head,
230                                src_span: internal_span,
231                            })
232                        }
233                    }
234                    other => Err(ShellError::OnlySupportsThisInputType {
235                        exp_input_type: "list, binary or range".into(),
236                        wrong_type: other.get_type().to_string(),
237                        dst_span: head,
238                        src_span: other.span(),
239                    }),
240                }
241            }
242            PipelineData::ByteStream(stream, ..) => {
243                if stream.type_().is_binary_coercible() {
244                    let span = stream.span();
245                    if let Some(mut reader) = stream.reader() {
246                        // Have to be a bit tricky here, but just consume into a VecDeque that we
247                        // shrink to fit each time
248                        const TAKE: u64 = 8192;
249                        let mut buf = VecDeque::with_capacity(rows + TAKE as usize);
250                        loop {
251                            let taken = std::io::copy(&mut (&mut reader).take(TAKE), &mut buf)
252                                .map_err(|err| IoError::new(err, span, None))?;
253                            if buf.len() > rows {
254                                buf.drain(..(buf.len() - rows));
255                            }
256                            if taken < TAKE {
257                                // This must be EOF.
258                                if return_single_element {
259                                    if !buf.is_empty() {
260                                        return Ok(
261                                            Value::int(buf[0] as i64, head).into_pipeline_data()
262                                        );
263                                    } else if strict_mode {
264                                        return Err(ShellError::AccessEmptyContent { span: head });
265                                    } else {
266                                        // There are no values, so return nothing instead of an error so
267                                        // that users can pipe this through 'default' if they want to.
268                                        return Ok(Value::nothing(head)
269                                            .into_pipeline_data_with_metadata(metadata));
270                                    }
271                                } else {
272                                    return Ok(Value::binary(buf, head).into_pipeline_data());
273                                }
274                            }
275                        }
276                    } else {
277                        Ok(PipelineData::empty())
278                    }
279                } else {
280                    Err(ShellError::OnlySupportsThisInputType {
281                        exp_input_type: "list, binary or range".into(),
282                        wrong_type: stream.type_().describe().into(),
283                        dst_span: head,
284                        src_span: stream.span(),
285                    })
286                }
287            }
288            PipelineData::Empty => Err(ShellError::OnlySupportsThisInputType {
289                exp_input_type: "list, binary or range".into(),
290                wrong_type: "null".into(),
291                dst_span: call.head,
292                src_span: call.head,
293            }),
294        }
295    }
296}
297
298#[cfg(test)]
299mod test {
300    use super::*;
301
302    #[test]
303    fn test_examples() -> nu_test_support::Result {
304        nu_test_support::test().examples(Last)
305    }
306}
shannon_nu_command/filters/last.rs

shannon_nu_command/filters/
last.rs
