googletest 0.14.2

// Copyright 2022 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

/// Counts the number of elements in `value`.
///
/// This uses [`Iterator::size_hint`] when that function returns an
/// unambiguous answer, i.e., the upper bound exists and the lower and upper
/// bounds agree. Otherwise it iterates through `value` and counts the
/// elements.
pub(crate) fn count_elements<ContainerT: IntoIterator>(value: ContainerT) -> usize {
    let iterator = value.into_iter();
    if let (lower, Some(higher)) = iterator.size_hint() {
        if lower == higher {
            return lower;
        }
    }
    iterator.count()
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::prelude::*;
    use crate::Result;

    #[test]
    fn count_elements_vec() -> Result<()> {
        verify_that!(count_elements(vec![1, 2, 3]), eq(3))
    }

    #[test]
    fn count_elements_with_imprecise_hint() -> Result<()> {
        struct FakeIterator;

        impl Iterator for FakeIterator {
            type Item = ();

            fn next(&mut self) -> Option<Self::Item> {
                None
            }

            fn size_hint(&self) -> (usize, Option<usize>) {
                (0, Some(123))
            }
        }

        verify_that!(count_elements(FakeIterator), eq(0))
    }

    #[test]
    fn count_elements_with_no_hint() -> Result<()> {
        struct FakeIterator;

        impl Iterator for FakeIterator {
            type Item = ();

            fn next(&mut self) -> Option<Self::Item> {
                None
            }

            fn size_hint(&self) -> (usize, Option<usize>) {
                (0, None)
            }
        }

        verify_that!(count_elements(FakeIterator), eq(0))
    }
}