From 46afb6bf501f4001c4bdb7bd2f2db7c466f95554 Mon Sep 17 00:00:00 2001
From: Juan Marin Noguera <juan@mnpi.eu>
Date: Tue, 25 Jul 2023 18:22:04 +0200
Subject: Solvned project from 2020

Note that Go didn't have generics back then.
---
 event.go | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 event.go

(limited to 'event.go')

diff --git a/event.go b/event.go
new file mode 100644
index 0000000..c4a687b
--- /dev/null
+++ b/event.go
@@ -0,0 +1,95 @@
+// Events mechanism for early iteration ending.
+//
+// Copyright (C) 2020 Juan Marín Noguera
+//
+// This file is part of Solvned.
+//
+// Solvned is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Lesser General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option) any 
+// later version.
+//
+// Solvned is distributed in the hope that it will be useful, but WITHOUT ANY 
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+// A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more
+// details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with Solvned. If not, see <https://www.gnu.org/licenses/>.
+
+package mned
+
+import "math"
+
+// An Event is a condition that can happen in a point in the solution of an
+// initial value problem and an associated action to take. The event happens
+// at the point of the solution where a given function changes its sign.
+//
+// The Cross function is a continuous function whose zeroes are the points
+// where the event happens. The Tolerance is the margin of error allowed; the
+// maximum absolute value of `Cross(p)` such that `p` is considered to be close
+// enough to an event to be passed to Action. The Action is to be called when
+// an occurrence of the event is found; it can use the point in some way and it
+// returns a boolean indicating whether the calculation should continue.
+type Event struct {
+	Cross     func(*Point) float64
+	Tolerance float64
+	Action    func(*Point) bool
+}
+
+// If e.Cross has a zero between p1 and p2, find such a zero or a point `p`
+// close enough to the zero that `|e.Cross(p)| < e.Tolerance`. To get the
+// intermediante points, the given interpolator is used.
+func (e *Event) FindPoint(i Interpolator, p1 *Point, p2 *Point) Point {
+	var min, max, mid Point
+	if p1.Time < p2.Time {
+		min, max = *p1, *p2
+	} else {
+		max, min = *p1, *p2
+	}
+	downwards := e.Cross(p1) > 0
+	for {
+		mid.Time = (min.Time + max.Time) / 2
+		mid.Value = i.FindValue(p1, p2, mid.Time)
+		value := e.Cross(&mid)
+		if math.Abs(value) < e.Tolerance {
+			return mid
+		}
+		if downwards {
+			if value > 0 {
+				min = mid
+			} else {
+				max = mid
+			}
+		} else {
+			if value > 0 {
+				max = mid
+			} else {
+				min = mid
+			}
+		}
+	}
+}
+
+type requiredAction struct {
+	index int
+	point Point
+}
+
+type requiredActions []requiredAction
+
+func (r requiredActions) Len() int {
+	return len(r)
+}
+
+func (r requiredActions) Less(i, j int) bool {
+	return r[i].point.Time < r[j].point.Time
+}
+
+func (r requiredActions) Swap(i, j int) {
+	r[i], r[j] = r[j], r[i]
+}
+
+func differentSign(f1 float64, f2 float64) bool {
+	return (f1 <= 0 && f2 >= 0) || (f1 >= 0 && f2 <= 0)
+}
-- 
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