Add itertools.combinations()

re: #1361

tests/snippets/stdlib_itertools.py

@@ -301,6 +301,7 @@ assert list(t[0]) == [1,2,3]
 assert list(t[0]) == []
 
 # itertools.product
+
 it = itertools.product([1, 2], [3, 4])
 assert (1, 3) == next(it)
 assert (1, 4) == next(it)
@@ -321,3 +322,25 @@ with assert_raises(TypeError):
     itertools.product(None)
 with assert_raises(TypeError):
     itertools.product([1, 2], repeat=None)
+
+# itertools.combinations
+
+it = itertools.combinations([1, 2, 3, 4], 2)
+assert list(it) == [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
+
+it = itertools.combinations([1, 2, 3], 1)
+assert list(it) == [(1,), (2,), (3,)]
+
+it = itertools.combinations([1, 2, 3], 2)
+assert next(it) == (1, 2)
+assert next(it) == (1, 3)
+assert next(it) == (2, 3)
+with assert_raises(StopIteration):
+    next(it)
+
+it = itertools.combinations([1, 2, 3], 4)
+with assert_raises(StopIteration):
+    next(it)
+
+with assert_raises(ValueError):
+    itertools.combinations([1, 2, 3, 4], -2)

vm/src/stdlib/itertools.rs

@@ -5,6 +5,7 @@ use std::ops::{AddAssign, SubAssign};
 use std::rc::Rc;
 
 use num_bigint::BigInt;
+use num_traits::sign::Signed;
 use num_traits::ToPrimitive;
 
 use crate::function::{Args, OptionalArg, PyFuncArgs};
@@ -848,6 +849,110 @@ impl PyItertoolsProduct {
     }
 }
 
+#[pyclass]
+#[derive(Debug)]
+struct PyItertoolsCombinations {
+    pool: Vec<PyObjectRef>,
+    indices: RefCell<Vec<usize>>,
+    r: Cell<usize>,
+    exhausted: Cell<bool>,
+}
+
+impl PyValue for PyItertoolsCombinations {
+    fn class(vm: &VirtualMachine) -> PyClassRef {
+        vm.class("itertools", "combinations")
+    }
+}
+
+#[pyimpl]
+impl PyItertoolsCombinations {
+    #[pyslot(new)]
+    fn tp_new(
+        cls: PyClassRef,
+        iterable: PyObjectRef,
+        r: PyIntRef,
+        vm: &VirtualMachine,
+    ) -> PyResult<PyRef<Self>> {
+        let iter = get_iter(vm, &iterable)?;
+        let pool = get_all(vm, &iter)?;
+
+        let r = r.as_bigint();
+        if r.is_negative() {
+            return Err(vm.new_value_error("r must be non-negative".to_string()));
+        }
+        let r = r.to_usize().unwrap();
+
+        let n = pool.len();
+
+        PyItertoolsCombinations {
+            pool,
+            indices: RefCell::new((0..r).collect()),
+            r: Cell::new(r),
+            exhausted: Cell::new(r > n),
+        }
+        .into_ref_with_type(vm, cls)
+    }
+
+    #[pymethod(name = "__iter__")]
+    fn iter(zelf: PyRef<Self>, _vm: &VirtualMachine) -> PyRef<Self> {
+        zelf
+    }
+
+    #[pymethod(name = "__next__")]
+    fn next(&self, vm: &VirtualMachine) -> PyResult {
+        // stop signal
+        if self.exhausted.get() {
+            return Err(new_stop_iteration(vm));
+        }
+
+        let n = self.pool.len();
+        let r = self.r.get();
+
+        let res = PyTuple::from(
+            self.pool
+                .iter()
+                .enumerate()
+                .filter(|(idx, _)| self.indices.borrow().contains(&idx))
+                .map(|(_, num)| num.clone())
+                .collect::<Vec<PyObjectRef>>(),
+        );
+
+        let mut indices = self.indices.borrow_mut();
+        let mut sentinel = false;
+
+        // Scan indices right-to-left until finding one that is not at its maximum (i + n - r).
+        let mut idx = r - 1;
+        loop {
+            if indices[idx] != idx + n - r {
+                sentinel = true;
+                break;
+            }
+
+            if idx != 0 {
+                idx -= 1;
+            } else {
+                break;
+            }
+        }
+        // If no suitable index is found, then the indices are all at
+        // their maximum value and we're done.
+        if !sentinel {
+            self.exhausted.set(true);
+        }
+
+        // Increment the current index which we know is not at its
+        // maximum.  Then move back to the right setting each index
+        // to its lowest possible value (one higher than the index
+        // to its left -- this maintains the sort order invariant).
+        indices[idx] += 1;
+        for j in idx + 1..r {
+            indices[j] = indices[j - 1] + 1;
+        }
+
+        Ok(res.into_ref(vm).into_object())
+    }
+}
+
 pub fn make_module(vm: &VirtualMachine) -> PyObjectRef {
     let ctx = &vm.ctx;
 
@@ -858,6 +963,9 @@ pub fn make_module(vm: &VirtualMachine) -> PyObjectRef {
 
     let compress = PyItertoolsCompress::make_class(ctx);
 
+    let combinations = ctx.new_class("combinations", ctx.object());
+    PyItertoolsCombinations::extend_class(ctx, &combinations);
+
     let count = ctx.new_class("count", ctx.object());
     PyItertoolsCount::extend_class(ctx, &count);
 
@@ -887,6 +995,7 @@ pub fn make_module(vm: &VirtualMachine) -> PyObjectRef {
         "accumulate" => accumulate,
         "chain" => chain,
         "compress" => compress,
+        "combinations" => combinations,
         "count" => count,
         "dropwhile" => dropwhile,
         "islice" => islice,