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#![deny(missing_docs)]
//! `flow_impl` is a trait that flow function implementations must implement in order
//! to be executed as part of a flow.
//!
use std::fmt::Debug;
use std::error;
use std::fmt;
use std::panic::{RefUnwindSafe, UnwindSafe};
use serde::{Serialize, Deserialize};
use serde_json::Value;

/// Implementations should return a value of type `RunAgain` to indicate if it should be
/// executed more times in the future.
pub type RunAgain = bool;
/// Use `RUN_AGAIN` to indicate that a function can be executed more times
pub const RUN_AGAIN: RunAgain = true;
/// Use `DONT_RUN_AGAIN` to indicate that a function should not be executed more times
pub const DONT_RUN_AGAIN: RunAgain = false;

/// A `RuntimeError` is an error that occurred during the running of a Function's `Implementation`
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum RuntimeError {
    /// There was an error with the inputs to the `Implementation`
    InputError(String),
    /// There was an error during the execution of the `Implementation`
    Execution(String)
}

impl fmt::Display for RuntimeError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{:?}", self)
    }
}

impl error::Error for RuntimeError {
    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        // Generic error, underlying cause isn't tracked.
        None
    }
}

/// An implementation runs with an array of inputs and returns a value (or null) and a
/// bool indicating if it should be ran again.
///
/// Any 'implementation' of a function must implement this trait
///
/// # Examples
///
/// Here is an example implementation of this trait:
///
/// ```
/// use flow_impl::{Implementation, RUN_AGAIN, RunAgain, RuntimeError};
/// use serde_json::Value;
/// use serde_json::json;
///
/// #[derive(Debug)]
/// pub struct Compare;
///
/// /*
///     A compare operator that takes two numbers and outputs the comparisons between them
/// */
/// impl Implementation for Compare {
///     fn run(&self, mut inputs: &[Value]) -> (Result<Option<Value>, RuntimeError>, RunAgain) {
///         let left = inputs[0].as_i64().unwrap();
///         let right = inputs[1].as_i64().unwrap();
///
///         let output = json!({
///                     "equal" : left == right,
///                     "lt" : left < right,
///                     "gt" : left > right,
///                     "lte" : left <= right,
///                     "gte" : left >= right
///                 });
///
///         (Ok(None), RUN_AGAIN)
///     }
/// }
/// ```
pub trait Implementation : RefUnwindSafe + UnwindSafe + Sync + Send + Debug {
    /// The `run` method is used to execute the implementation
    fn run(&self, inputs: &[Value]) -> (Result<Option<Value>, RuntimeError>, RunAgain);
}