/-
Copyright (c) 2021 Matt Kempster. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Matt Kempster
-/
import Mathlib.Geometry.Euclidean.Triangle

/-!
# Freek № 57: Heron's Formula

This file proves Theorem 57 from the [100 Theorems List](https://www.cs.ru.nl/~freek/100/),
also known as Heron's formula, which gives the area of a triangle based only on its three sides'
lengths.

## References

* https://en.wikipedia.org/wiki/Herons_formula

-/


open Real EuclideanGeometry

open scoped Real EuclideanGeometry

namespace Theorems100

local notation "√" => Real.sqrt

variable {V : Type*} {P : Type*} [NormedAddCommGroup V] [InnerProductSpace ℝ V] [MetricSpace P]
  [NormedAddTorsor V P]

/-- **Heron's formula**: The area of a triangle with side lengths `a`, `b`, and `c` is
  `√(s * (s - a) * (s - b) * (s - c))` where `s = (a + b + c) / 2` is the semiperimeter.
  We show this by equating this formula to `a * b * sin γ`, where `γ` is the angle opposite
  the side `c`.
-/
theorem heron {p₁ p₂ p₃ : P} (h1 : p₁ ≠ p₂) (h2 : p₃ ≠ p₂) :
    let a := dist p₁ p₂
    let b := dist p₃ p₂
    let c := dist p₁ p₃
    let s := (a + b + c) / 2
    1 / 2 * a * b * sin (∠ p₁ p₂ p₃) = √(s * (s - a) * (s - b) * (s - c)) := by
  intro a b c s
  let γ := ∠ p₁ p₂ p₃
  obtain := (dist_pos.mpr h1).ne', (dist_pos.mpr h2).ne'
  have cos_rule : cos γ = (a * a + b * b - c * c) / (2 * a * b) := by
    simp [field, a, b, c, γ, mul_comm,
      dist_sq_eq_dist_sq_add_dist_sq_sub_two_mul_dist_mul_dist_mul_cos_angle p₁ p₂ p₃]
  let numerator := (2 * a * b) ^ 2 - (a * a + b * b - c * c) ^ 2
  let denominator := (2 * a * b) ^ 2
  have split_to_frac : ↑1 - cos γ ^ 2 = numerator / denominator := by
    simp [field, numerator, denominator, cos_rule]
  have numerator_nonneg : 0 ≤ numerator := by
    have frac_nonneg : 0 ≤ numerator / denominator :=
      (sub_nonneg.mpr (cos_sq_le_one γ)).trans_eq split_to_frac
    rcases div_nonneg_iff.mp frac_nonneg with h | h
    · exact h.left
    · simpa [numerator, denominator, a, b, c, h1, h2] using le_antisymm h.right (sq_nonneg _)
  have ab2_nonneg : 0 ≤ 2 * a * b := by positivity
  calc
    1 / 2 * a * b * sin γ = 1 / 2 * a * b * (√numerator / √denominator) := by
      rw [sin_eq_sqrt_one_sub_cos_sq, split_to_frac, sqrt_div numerator_nonneg] <;>
        simp [γ, angle_nonneg, angle_le_pi]
    _ = 1 / 4 * √((2 * a * b) ^ 2 - (a * a + b * b - c * c) ^ 2) := by
      simp (disch := positivity) [field, numerator, denominator, -mul_eq_mul_left_iff]; ring
    _ = ↑1 / ↑4 * √(s * (s - a) * (s - b) * (s - c) * ↑4 ^ 2) := by simp only [s]; ring_nf
    _ = √(s * (s - a) * (s - b) * (s - c)) := by
      rw [sqrt_mul', sqrt_sq, div_mul_eq_mul_div, one_mul, mul_div_cancel_right₀] <;> norm_num

end Theorems100