prove 0.1.5

prove struct
Documentation 
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// Copyright 2019 YechaoLi
//
// 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.

use std::borrow::Cow;

#[derive(Debug, Clone)]
pub struct Length {
    pub min: Option<u64>,
    pub max: Option<u64>,
    pub eq: Option<u64>,
}

#[cfg(feature = "proc")]
impl quote::ToTokens for Length {
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
        use quote::quote;

        let min = if let Some(min) = self.min {
            quote!(Some(#min))
        } else {
            quote!(None)
        };
        let max = if let Some(max) = self.max {
            quote!(Some(#max))
        } else {
            quote!(None)
        };
        let eq = if let Some(eq) = self.eq {
            quote!(Some(#eq))
        } else {
            quote!(None)
        };

        tokens.extend(quote! {
            (::prove::length::Length {
                min: #min,
                max: #max,
                eq: #eq,
            })
        });
    }
}

impl Length {
    pub fn validate<T>(&self, val: T) -> bool
    where
        T: HasLen,
    {
        if let Some(len) = val.length() {
            if let Some(eq) = self.eq {
                return len == eq;
            }
            if let Some(min) = self.min {
                if len < min {
                    return false;
                }
            }
            if let Some(max) = self.max {
                if len > max {
                    return false;
                }
            }
        }
        true
    }
}

pub trait HasLen {
    fn length(&self) -> Option<u64>;
}

impl HasLen for String {
    fn length(&self) -> Option<u64> {
        Some(self.chars().count() as u64)
    }
}

impl<'a> HasLen for &'a String {
    fn length(&self) -> Option<u64> {
        Some(self.chars().count() as u64)
    }
}

impl<'a> HasLen for &'a str {
    fn length(&self) -> Option<u64> {
        Some(self.chars().count() as u64)
    }
}

impl<'a> HasLen for Cow<'a, str> {
    fn length(&self) -> Option<u64> {
        Some(self.len() as u64)
    }
}

impl<T> HasLen for Vec<T> {
    fn length(&self) -> Option<u64> {
        Some(self.len() as u64)
    }
}

impl<'a, T> HasLen for &'a Vec<T> {
    fn length(&self) -> Option<u64> {
        Some(self.len() as u64)
    }
}

impl<T> HasLen for Option<T>
where
    T: HasLen,
{
    fn length(&self) -> Option<u64> {
        match self {
            None => None,
            Some(v) => v.length(),
        }
    }
}

impl<'a, T> HasLen for &'a Option<T>
where
    T: HasLen,
{
    fn length(&self) -> Option<u64> {
        match self {
            None => None,
            Some(v) => v.length(),
        }
    }
}