For problem statement at 1000-1999/1800-1899/1860-1869/1868/problemD.txt this is a correct solution, but verifier at 1000-1999/1800-1899/1860-1869/1868/verifierD.go ends with case 71 failed: reference failed: runtime error: exit status 2
panic: runtime error: index out of range [-1]

goroutine 1 [running]:
main.solve.func2(...)
	/home/ubuntu/codeforces/1000-1999/1800-1899/1860-1869/1868/1868D.go:47
main.solve()
	/home/ubuntu/codeforces/1000-1999/1800-1899/1860-1869/1868/1868D.go:134 +0xd24
main.main()
	/home/ubuntu/codeforces/1000-1999/1800-1899/1860-1869/1868/1868D.go:153 +0xb0

input:
1
6
3 0 3 2 3 1
exit status 1 can you fix the verifier? package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
)

type Node struct {
	id  int
	deg int
}

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var t int
	if _, err := fmt.Fscan(reader, &t); err != nil {
		return
	}

	for tc := 0; tc < t; tc++ {
		var n int
		fmt.Fscan(reader, &n)

		d := make([]int, n)
		sumD := 0
		nodes := make([]Node, n)
		for i := 0; i < n; i++ {
			fmt.Fscan(reader, &d[i])
			sumD += d[i]
			nodes[i] = Node{id: i + 1, deg: d[i]}
		}

		if sumD != 2*n {
			fmt.Fprintln(writer, "No")
			continue
		}

		sort.Slice(nodes, func(i, j int) bool {
			return nodes[i].deg > nodes[j].deg
		})

		M := 0
		M3 := 0
		for i := 0; i < n; i++ {
			if nodes[i].deg >= 2 {
				M++
			}
			if nodes[i].deg >= 3 {
				M3++
			}
		}

		if M == n {
			fmt.Fprintln(writer, "Yes")
			for i := 0; i < n; i++ {
				fmt.Fprintln(writer, nodes[i].id, nodes[(i+1)%n].id)
			}
			continue
		}

		prefixD := make([]int, M+1)
		for i := 0; i < M; i++ {
			prefixD[i+1] = prefixD[i] + nodes[i].deg
		}

		foundC := -1
		foundK := -1

		for C := 2; C <= M3; C++ {
			k := M / C
			if k == 0 {
				continue
			}
			if k == 1 {
				if M == C {
					foundC = C
					foundK = k
					break
				}
				continue
			}

			idx := C
			OPrev := prefixD[C] - 2*C
			possible := true
			rem := M - k*C

			for step := 1; step < k; step++ {
				if OPrev < C {
					possible = false
					break
				}
				canAdd := OPrev - C
				add := rem
				if canAdd < add {
					add = canAdd
				}
				LSize := C + add
				rem -= add

				OCurr := prefixD[idx+LSize] - prefixD[idx] - LSize
				OPrev = OCurr
				idx += LSize
			}

			if possible && rem == 0 {
				foundC = C
				foundK = k
				break
			}
		}

		if foundC == -1 {
			fmt.Fprintln(writer, "No")
		} else {
			fmt.Fprintln(writer, "Yes")
			LSizes := make([]int, foundK)
			for i := range LSizes {
				LSizes[i] = foundC
			}
			rem := M - foundK*foundC
			idx := foundC
			OPrev := prefixD[foundC] - 2*foundC

			for step := 1; step < foundK; step++ {
				canAdd := OPrev - foundC
				add := rem
				if canAdd < add {
					add = canAdd
				}
				LSizes[step] += add
				rem -= add
				OCurr := prefixD[idx+LSizes[step]] - prefixD[idx] - LSizes[step]
				OPrev = OCurr
				idx += LSizes[step]
			}

			outDeg := make([]int, M)
			for i := 0; i < foundC; i++ {
				outDeg[i] = nodes[i].deg - 2
			}
			for i := foundC; i < M; i++ {
				outDeg[i] = nodes[i].deg - 1
			}

			for i := 0; i < foundC; i++ {
				fmt.Fprintln(writer, nodes[i].id, nodes[(i+1)%foundC].id)
			}

			layers := make([][]int, foundK)
			nodeIdx := 0
			for i, sz := range LSizes {
				layers[i] = make([]int, sz)
				for j := 0; j < sz; j++ {
					layers[i][j] = nodeIdx
					nodeIdx++
				}
			}

			for i := 0; i < foundK-1; i++ {
				prevLayer := layers[i]
				currLayer := layers[i+1]

				for j := 0; j < foundC; j++ {
					fmt.Fprintln(writer, nodes[prevLayer[j]].id, nodes[currLayer[j]].id)
					outDeg[prevLayer[j]] -= 1
				}

				parentIdx := 0
				for j := foundC; j < len(currLayer); j++ {
					for parentIdx < len(prevLayer) && outDeg[prevLayer[parentIdx]] == 0 {
						parentIdx++
					}
					p := prevLayer[parentIdx]
					fmt.Fprintln(writer, nodes[p].id, nodes[currLayer[j]].id)
					outDeg[p] -= 1
				}
			}

			leafIdx := M
			for i := 0; i < M; i++ {
				for outDeg[i] > 0 {
					fmt.Fprintln(writer, nodes[i].id, nodes[leafIdx].id)
					outDeg[i] -= 1
					leafIdx++
				}
			}
		}
	}
}