use serde::de::{self, SeqAccess, Visitor};
use serde::{Deserialize, Deserializer};

use std::marker::PhantomData;
use std::{cmp, fmt};

#[derive(Deserialize)]
struct Outer {
    id: String,

    // Deserialize this field by computing the maximum value of a sequence
    // (JSON array) of values.
    #[serde(deserialize_with = "deserialize_max")]
    // Despite the struct field being named `max_value`, it is going to come
    // from a JSON field called `values`.
    #[serde(rename(deserialize = "values"))]
    max_value: u64,
}

/// Deserialize the maximum of a sequence of values. The entire sequence
/// is not buffered into memory as it would be if we deserialize to Vec<T>
/// and then compute the maximum later.
///
/// This function is generic over T which can be any type that implements
/// Ord. Above, it is used with T=u64.
fn deserialize_max<'de, T, D>(deserializer: D) -> Result<T, D::Error>
where
    T: Deserialize<'de> + Ord,
    D: Deserializer<'de>,
{
    struct MaxVisitor<T>(PhantomData<fn() -> T>);

    impl<'de, T> Visitor<'de> for MaxVisitor<T>
    where
        T: Deserialize<'de> + Ord,
    {
        /// Return type of this visitor. This visitor computes the max of a
        /// sequence of values of type T, so the type of the maximum is T.
        type Value = T;

        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str("a nonempty sequence of numbers")
        }

        fn visit_seq<S>(self, mut seq: S) -> Result<T, S::Error>
        where
            S: SeqAccess<'de>,
        {
            // Start with max equal to the first value in the seq.
            let mut max = seq.next_element()?.ok_or_else(||
                // Cannot take the maximum of an empty seq.
                de::Error::custom("no values in seq when looking for maximum"))?;

            // Update the max while there are additional values.
            while let Some(value) = seq.next_element()? {
                max = cmp::max(max, value);
            }

            Ok(max)
        }
    }

    // Create the visitor and ask the deserializer to drive it. The
    // deserializer will call visitor.visit_seq() if a seq is present in
    // the input data.
    let visitor = MaxVisitor(PhantomData);
    deserializer.deserialize_seq(visitor)
}

fn main() {
    let j = r#"
        {
          "id": "demo-deserialize-max",
          "values": [
            256,
            100,
            384,
            314,
            271
          ]
        }
    "#;

    let out: Outer = serde_json::from_str(j).unwrap();

    // Prints "max value: 384"
    println!("max value: {}", out.max_value);
}