From 07442255f6ecd2def215c332accb5792be2158e3 Mon Sep 17 00:00:00 2001
From: Julien Dessaux
Date: Wed, 15 Dec 2021 17:33:01 +0100
Subject: Added solutions for 15th day: Chiton shortest path exercises

---
 2021/15/first.go | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 2021/15/first.go

(limited to '2021/15/first.go')

diff --git a/2021/15/first.go b/2021/15/first.go
new file mode 100644
index 0000000..54b1344
--- /dev/null
+++ b/2021/15/first.go
@@ -0,0 +1,99 @@
+package main
+
+import (
+	"bufio"
+	"container/heap"
+	"fmt"
+	"os"
+)
+
+type Node struct {
+	x, y, cost int
+	index      int
+}
+type NodeQueue []*Node
+
+func (pq NodeQueue) Len() int           { return len(pq) }
+func (pq NodeQueue) Less(i, j int) bool { return pq[i].cost < pq[j].cost }
+func (pq NodeQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+	pq[i].index = i
+	pq[j].index = j
+}
+func (pq *NodeQueue) Push(x interface{}) {
+	item := x.(*Node)
+	item.index = len(*pq)
+	*pq = append(*pq, item)
+}
+func (pq *NodeQueue) Pop() interface{} {
+	p := *pq
+	l := len(p) - 1
+	item := p[l]
+	p[l] = nil
+	*pq = p[0:l]
+	return item
+}
+func (pq *NodeQueue) Update(n *Node, cost int) {
+	n.cost = cost
+	heap.Fix(pq, n.index)
+}
+
+var (
+	grid  [][]byte
+	costs [][]Node
+	nq    = make(NodeQueue, 1)
+	size  int
+)
+
+func calculateTentativeDistance(x, y, cost int) {
+	if x < 0 || x >= size || y < 0 || y >= size {
+		return
+	}
+	cost += int(grid[y][x])
+	if costs[y][x].cost == 0 {
+		costs[y][x].cost = cost
+		heap.Push(&nq, &costs[y][x])
+	} else if cost < costs[y][x].cost {
+		nq.Update(&costs[y][x], cost)
+	}
+}
+func main() {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	line := scanner.Text()
+	size = len(line)
+
+	y := 0
+	for {
+		l := make([]byte, size)
+		for i := 0; i < size; i++ {
+			l[i] = line[i] - '0'
+		}
+		grid = append(grid, l)
+		c := make([]Node, size)
+		for j := 0; j < size; j++ {
+			c[j] = Node{x: j, y: y}
+		}
+		costs = append(costs, c)
+		if !scanner.Scan() {
+			break
+		}
+		line = scanner.Text()
+		y++
+	}
+	grid[0][0] = 0
+	nq[0] = &costs[0][0]
+	heap.Init(&nq)
+
+	for {
+		n := heap.Pop(&nq).(*Node)
+		if n.x == size-1 && n.y == size-1 {
+			fmt.Println(n.cost)
+			break
+		}
+		calculateTentativeDistance(n.x-1, n.y, n.cost)
+		calculateTentativeDistance(n.x+1, n.y, n.cost)
+		calculateTentativeDistance(n.x, n.y-1, n.cost)
+		calculateTentativeDistance(n.x, n.y+1, n.cost)
+	}
+}
-- 
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