// (C) Copyright 2019-2020 Hewlett Packard Enterprise Development LP

use std::collections::HashSet;
use std::fmt;
use std::iter::FromIterator;

use super::expression::{Expression, BExpression};
use super::misc::{Span, Subquery};
use super::return_value::{LabelSetOp, ReturnKind, ReturnValue, strs_to_set};

/// Aggregation operator types, namely "by" and "without"
#[derive(Debug, PartialEq, Eq, Clone)]
pub enum AggregationOp {
  By,
  Without
}

impl fmt::Display for AggregationOp {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    match self {
      AggregationOp::By => write!(f, "by"),
      AggregationOp::Without => write!(f, "without")
    }
  }
}

/// A function aggregation clause.
///
/// Not all functions can be aggregated, but this library currently does not
/// attempt to validate this sort of usage.
#[derive(Debug, PartialEq, Eq, Clone)]
pub struct Aggregation {
  pub op: AggregationOp,
  pub labels: Vec<String>,

  pub span: Option<Span>
}

impl Aggregation {
  pub fn new(op: AggregationOp) -> Self {
    Aggregation {
      op, labels: vec![], span: None
    }
  }

  pub fn by() -> Self {
    Aggregation::new(AggregationOp::By)
  }

  pub fn without() -> Self {
    Aggregation::new(AggregationOp::Without)
  }

  /// Replaces this Aggregation's operator
  pub fn op(mut self, op: AggregationOp) -> Self {
    self.op = op;
    self
  }

  /// Adds a label key to this Aggregation
  pub fn label<S: Into<String>>(mut self, label: S) -> Self {
    self.labels.push(label.into());
    self
  }

  /// Replaces this Aggregation's labels with the given set
  pub fn labels(mut self, labels: &[&str]) -> Self {
    self.labels = labels.iter().map(|l| (*l).to_string()).collect();
    self
  }

  /// Clear this Matching's set of labels
  pub fn clear_labels(mut self) -> Self {
    self.labels.clear();
    self
  }

  pub fn span<S: Into<Span>>(mut self, span: S) -> Self {
    self.span = Some(span.into());
    self
  }
}

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

    if !self.labels.is_empty() {
      write!(f, " (")?;

      for (i, label) in self.labels.iter().enumerate() {
        if i > 0 {
          write!(f, ", ")?;
        }

        write!(f, "{}", label)?;
      }

      write!(f, ")")?;
    }

    Ok(())
  }
}

/// A function call.
///
/// Note that function names, arguments, argument types, and return types are
/// not validated.
#[derive(Debug, PartialEq, Clone)]
pub struct Function {
  pub name: String,
  pub args: Vec<BExpression>,

  pub aggregation: Option<Aggregation>,

  pub subquery: Option<Subquery>,

  pub span: Option<Span>
}

/// Given a Prometheus function name, returns the argument index that determines
/// the set of resulting labels
fn arg_index_for_function(name: &str) -> Option<usize> {
  Some(match name.to_lowercase().as_str() {
    "time" => return None,

    "histogram_quantile" => 1,
    "quantile_over_time" => 1,

    // almost all functions use the first arg (often by only accepting 1 arg)
    _ => 0
  })
}

/// Determines if a given function converts a range vector to an instance vector
///
/// Note that `_over_time` functions do not affect labels, unlike their regular
/// counterparts
fn is_aggregation_over_time(name: &str) -> bool {
  match name.to_lowercase().as_str() {
    // all the _over_time functions ...
    "avg_over_time" | "min_over_time" | "max_over_time" => true,
    "sum_over_time" | "count_over_time" | "quantile_over_time" => true,
    "stddev_over_time" | "stdvar_over_time" => true,

    // and also a few more
    "delta" | "deriv" | "idelta" | "increase" | "predict_linear" => true,
    "irate" | "rate" | "resets" => true,

    _ => false
  }
}

/// Determines if a given function is an aggregation function
fn is_aggregation(name: &str) -> bool {
  match name.to_lowercase().as_str() {
    "sum" | "min" | "max" | "avg" | "stddev" | "stdvar" | "count" => true,
    "count_values" | "bottomk" | "topk" | "quantile" => true,
    _ => false
  }
}

impl Function {
  pub fn new<S: Into<String>>(name: S) -> Function {
    Function {
      name: name.into(),
      args: vec![],
      aggregation: None,
      subquery: None,
      span: None
    }
  }

  /// Replaces this Function's name with the given string
  ///
  /// Note that the new name is not validated.
  pub fn name<S: Into<String>>(mut self, name: S) -> Self {
    self.name = name.into();
    self
  }

  /// Adds the given expression as new argument to this function 
  pub fn arg(mut self, arg: Expression) -> Self {
    self.args.push(Box::new(arg));
    self
  }

  /// Sets this Function's aggregation clause
  pub fn aggregation(mut self, aggregation: Aggregation) -> Self {
    self.aggregation = Some(aggregation);
    self
  }

  /// Clears this Function's aggregation clause, if any 
  pub fn clear_aggregation(mut self) -> Self {
    self.aggregation = None;
    self
  }

  pub fn subquery(mut self, subquery: Subquery) -> Self {
    self.subquery = Some(subquery);
    self
  }

  pub fn clear_subquery(mut self) -> Self {
    self.subquery = None;
    self
  }

  pub fn span<S: Into<Span>>(mut self, span: S) -> Self {
    self.span = Some(span.into());
    self
  }

  /// Wraps this function in an Expression
  pub fn wrap(self) -> Expression {
    Expression::Function(self)
  }

  pub fn return_value(&self) -> ReturnValue {
    // functions generally pass through labels of one of their arguments, with
    // some exceptions

    // determine the arg to pass through
    let labels_arg_index = match arg_index_for_function(&self.name) {
      Some(index) => index,

      // only `time(...)` accepts no arguments and always returns a scalar
      None => return ReturnValue {
        kind: ReturnKind::Scalar,
        label_ops: vec![LabelSetOp::clear(self.clone().wrap(), self.span)]
      }
    };

    // if the function call is invalid, bail with unknowns
    let labels_arg = match self.args.get(labels_arg_index) {
      Some(arg) => arg,
      None => return ReturnValue {
        kind: ReturnKind::unknown(
          format!(
            "function call {}(...) is missing a required argument at index {}",
            self.name, labels_arg_index
          ),
          self.clone().wrap()
        ),

        // we can't predict what will happen with labels, but we can at least
        // try to output _something_
        label_ops: vec![]
      }
    };

    let arg_return = labels_arg.return_value();
    
    let mut kind = arg_return.kind;
    let mut label_ops = arg_return.label_ops;

    if is_aggregation_over_time(&self.name) {
      if let ReturnKind::RangeVector = kind {
        kind = ReturnKind::InstantVector;
      } else {
        // invalid arg
        kind = ReturnKind::unknown(
          format!("aggregation over time is not valid with expression returning {:?}", kind),

          // show the label arg as the cause
          // doesn't follow the usual pattern of showing the parent, but would
          // otherwise be ambiguous with multiple args
          *labels_arg.clone()
        );
      }
    }

    // aggregation functions reset labels unless they have an aggregation clause
    // (handled below)
    let is_agg = is_aggregation(&self.name);

    // aggregations turn range vectors into instant vectors
    if let Some(agg) = &self.aggregation {
      match agg.op {
        AggregationOp::By => {
          // by (...) resets labels to only those in the by clause
          label_ops.push(LabelSetOp::clear(self.clone().wrap(), agg.span));
          label_ops.push(LabelSetOp::append(
            self.clone().wrap(),
            agg.span,
            HashSet::from_iter(agg.labels.iter().cloned())
          ));
        },
        AggregationOp::Without => label_ops.push(LabelSetOp::remove(
          self.clone().wrap(),
          agg.span,
          HashSet::from_iter(agg.labels.iter().cloned()),
        ))
      }
    } else if is_agg {
      // an aggregation function with no aggregation clause clears all labels
      label_ops.push(LabelSetOp::clear(self.clone().wrap(), self.span));
    }
    
    // handle special cases with functions that mutate labels
    match self.name.to_lowercase().as_str() {
      // creats a new label with contents of others, leaves them intact
      "label_join" => if let Some(expr) = self.args.get(1) {
        if let Some(s) = expr.as_str() {
          label_ops.push(LabelSetOp::append(
            self.clone().wrap(),
            self.span,
            strs_to_set(&[s])
          ));
        }
      },

      // note: does not remove src_label
      "label_replace" => if let Some(expr) = self.args.get(1) {
        if let Some(s) = expr.as_str() {
          label_ops.push(LabelSetOp::append(
            self.clone().wrap(),
            self.span,
            strs_to_set(&[s])
          ));
        }
      },

      // per docs, input vector must have an `le` label
      "histogram_quantile" => label_ops.push(LabelSetOp::append(
        self.clone().wrap(),
        self.span,
        strs_to_set(&["le"])
      )),

      // "vector" => // no-op, its input expr is a scalar anyway

      _ => ()
    }

    // subqueries turn instant vectors into ranges
    if self.subquery.is_some() {
      kind = match kind {
        ReturnKind::InstantVector => ReturnKind::RangeVector,
        _ => ReturnKind::unknown(
          format!("subquery on inner expression returning {:?} is invalid", kind),
          self.clone().wrap()
        )
      };
    }

    ReturnValue { kind, label_ops }
  }
}

impl fmt::Display for Function {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    write!(f, "{}(", self.name)?;
    
    for (i, arg) in self.args.iter().enumerate() {
      if i > 0 {
        write!(f, ", ")?;
      }

      write!(f, "{}", arg)?;
    }

    write!(f, ")")?;

    if let Some(agg) = &self.aggregation {
      write!(f, " {}", agg)?;
    }

    if let Some(subquery) = &self.subquery {
      write!(f, "{}", subquery)?;
    }

    Ok(())
  }
}