description: An important meeting is to be held and there are exactly n people invited. At any moment, any two people can step back and talk in private. The same two people can talk several (as many as they want) times per meeting.
  
  Each person has limited sociability. The sociability of the i-th person is a non-negative integer a_i. This means that after exactly a_i talks this person leaves the meeting (and does not talk to anyone else anymore). If a_i = 0, the i-th person leaves the meeting immediately after it starts.
  
  A meeting is considered most productive if the maximum possible number of talks took place during it.
  
  You are given an array of sociability a, determine which people should talk to each other so that the total number of talks is as large as possible.
  
  Input
  
  The first line contains an integer t (1 ≤ t ≤ 1000) — the number of test cases.
  
  The next 2t lines contain descriptions of the test cases.
  
  The first line of each test case description contains an integer n (2 ≤ n ≤ 2 ⋅ 10^5) —the number of people in the meeting. The second line consists of n space-separated integers a_1, a_2, ..., a_n (0 ≤ a_i ≤ 2 ⋅ 10^5) — the sociability parameters of all people. 
  
  It is guaranteed that the sum of n over all test cases does not exceed 2 ⋅ 10^5. It is also guaranteed that the sum of all a_i (over all test cases and all i) does not exceed 2 ⋅ 10^5.
  
  Output
  
  Print t answers to all test cases.
  
  On the first line of each answer print the number k — the maximum number of talks possible in a meeting.
  
  On each of the next k lines print two integers i and j (1 ≤ i, j ≤ n and i ≠ j) — the numbers of people who will have another talk.
  
  If there are several possible answers, you may print any of them.
  
  Example
  
  Input
  
  
  8
  2
  2 3
  3
  1 2 3
  4
  1 2 3 4
  3
  0 0 2
  2
  6 2
  3
  0 0 2
  5
  8 2 0 1 1
  5
  0 1 0 0 6
  
  
  Output
  
  
  2
  1 2
  1 2
  3
  1 3
  2 3
  2 3
  5
  1 3
  2 4
  2 4
  3 4
  3 4
  0
  2
  1 2
  1 2
  0
  4
  1 2
  1 5
  1 4
  1 2
  1
  5 2
solution: import java.io.*;
  import java.math.*;
  import java.util.*;
  import java.util.stream.*;
  import java.lang.management.*;
  import static java.lang.Math.*;
  
  @SuppressWarnings("unchecked")
  public class P1579D {
  
    class Man implements Comparable<Man> {
      int n, a;
  
      public Man(int n, int a) {
        this.n = n;
        this.a = a;
      }
  
      @Override public int compareTo(Man m) {
        return ((a != m.a) ? (a - m.a) : (n - m.n));
      }
  
      @Override public String toString() {
        return ("[" + n + ", " + a + "]");
      }
    }
  
    public void run() throws Exception {
      TreeSet<Man> ms = new TreeSet<>();
  
  nextT:
      for (int T = nextInt(); T > 0; T--) {
        ms.clear();
        int t = 0;
        StringBuilder sb = new StringBuilder();
  
        for (int i = 1, n = nextInt(); i <= n; ms.add(new Man(i, nextInt())), i++);
        while (!ms.isEmpty() && (ms.first().a == 0)) { ms.pollFirst(); }
  
        for (; ms.size() >= 2; t++) {
          Man a = ms.pollLast(), b = ms.pollLast();
          if (a.a > 1) { ms.add(new Man(a.n, a.a - 1)); }
          if (b.a > 1) { ms.add(new Man(b.n, b.a - 1)); }
          sb.append(a.n).append(' ').append(b.n).append('\n');
        }
  
        print(t + "\n" + sb);
      }
    }
  
    public static void main(String... args) throws Exception {
      br = new BufferedReader(new InputStreamReader(System.in));
      pw = new PrintWriter(new BufferedOutputStream(System.out));
      new P1579D().run();
      br.close();
      pw.close();
      System.err.println("\n[Time : " + (System.currentTimeMillis() - startTime) + " ms]");
      long gct = 0, gcc = 0;
      for (GarbageCollectorMXBean garbageCollectorMXBean : ManagementFactory.getGarbageCollectorMXBeans()) {
        gct += garbageCollectorMXBean.getCollectionTime();
        gcc += garbageCollectorMXBean.getCollectionCount();
      }
      System.err.println("[GC time : " + gct + " ms, count = " + gcc + "]");
      System.err.println("[JIT time : " + ManagementFactory.getCompilationMXBean().getTotalCompilationTime() + " ms]");
    }
  
    static long startTime = System.currentTimeMillis();
    static BufferedReader br;
    static PrintWriter pw;
    StringTokenizer stok;
  
    String nextToken() throws IOException {
      while (stok == null || !stok.hasMoreTokens()) {
        String s = br.readLine();
        if (s == null) { return null; }
        stok = new StringTokenizer(s);
      }
      return stok.nextToken();
    }
  
    void print(byte b) { print("" + b); }
    void print(int i) { print("" + i); }
    void print(long l) { print("" + l); }
    void print(double d) { print("" + d); }
    void print(char c) { print("" + c); }
    void print(Object o) {
      if (o instanceof int[]) { print(Arrays.toString((int [])o));
      } else if (o instanceof long[]) { print(Arrays.toString((long [])o));
      } else if (o instanceof char[]) { print(Arrays.toString((char [])o));
      } else if (o instanceof byte[]) { print(Arrays.toString((byte [])o));
      } else if (o instanceof short[]) { print(Arrays.toString((short [])o));
      } else if (o instanceof boolean[]) { print(Arrays.toString((boolean [])o));
      } else if (o instanceof float[]) { print(Arrays.toString((float [])o));
      } else if (o instanceof double[]) { print(Arrays.toString((double [])o));
      } else if (o instanceof Object[]) { print(Arrays.toString((Object [])o));
      } else { print("" + o); }
    }
    void printsp(int [] a) { for (int i = 0, n = a.length; i < n; print(a[i] + " "), i++); }
    void printsp(long [] a) { for (int i = 0, n = a.length; i < n; print(a[i] + " "), i++); }
    void print(String s) { pw.print(s); }
    void println() { println(""); }
    void println(byte b) { println("" + b); }
    void println(int i) { println("" + i); }
    void println(long l) { println("" + l); }
    void println(double d) { println("" + d); }
    void println(char c) { println("" + c); }
    void println(Object o) { print(o); println(); }
    void println(String s) { pw.println(s); }
    int nextInt() throws IOException { return Integer.parseInt(nextToken()); }
    long nextLong() throws IOException { return Long.parseLong(nextToken()); }
    double nextDouble() throws IOException { return Double.parseDouble(nextToken()); }
    char nextChar() throws IOException { return (char) (br.read()); }
    String next() throws IOException { return nextToken(); }
    String nextLine() throws IOException { return br.readLine(); }
    int [] readInt(int size) throws IOException {
      int [] array = new int [size];
      for (int i = 0; i < size; i++) { array[i] = nextInt(); }
      return array;
    }
    long [] readLong(int size) throws IOException {
      long [] array = new long [size];
      for (int i = 0; i < size; i++) { array[i] = nextLong(); }
      return array;
    }
    double [] readDouble(int size) throws IOException {
      double [] array = new double [size];
      for (int i = 0; i < size; i++) { array[i] = nextDouble(); }
      return array;
    }
    String [] readLines(int size) throws IOException {
      String [] array = new String [size];
      for (int i = 0; i < size; i++) { array[i] = nextLine(); }
      return array;
    }
  
    int gcd(int a, int b) {
      if (a == 0) return Math.abs(b); if (b == 0) return Math.abs(a);
      a = Math.abs(a); b = Math.abs(b);
      int az = Integer.numberOfTrailingZeros(a), bz = Integer.numberOfTrailingZeros(b);
      a >>>= az; b >>>= bz;
      while (a != b) {
        if (a > b) { a -= b; a >>>= Integer.numberOfTrailingZeros(a); }
              else { b -= a; b >>>= Integer.numberOfTrailingZeros(b); }
      }
      return (a << Math.min(az, bz));
    }
  
    long gcd(long a, long b) {
      if (a == 0) return Math.abs(b); if (b == 0) return Math.abs(a);
      a = Math.abs(a); b = Math.abs(b);
      int az = Long.numberOfTrailingZeros(a), bz = Long.numberOfTrailingZeros(b);
      a >>>= az; b >>>= bz;
      while (a != b) {
        if (a > b) { a -= b; a >>>= Long.numberOfTrailingZeros(a); }
              else { b -= a; b >>>= Long.numberOfTrailingZeros(b); }
      }
      return (a << Math.min(az, bz));
    }
  
    boolean isPrime(int v) {
      if (v <= 1) { return false; }
      if (v <= 3) { return true; }
      if (((v & 1) == 0) || ((v % 3) == 0)) { return false; }
      for (int m = 5, n = (int)sqrt(v); m <= n; m += 6) { if (((v % m) == 0) || ((v % (m + 2)) == 0)) { return false; } }
      return true;
    }
  
    int [] rshuffle(int [] a) { // RANDOM shuffle
      Random r = new Random();
      for (int i = a.length - 1, j, t; i >= 0; j = r.nextInt(a.length), t = a[i], a[i] = a[j], a[j] = t, i--);
      return a;
    }
  
    int [] qshuffle(int [] a) { // QUICK shuffle
      int m = new Random().nextInt(10) + 2;
      for (int i = 0, n = a.length, j = m % n, t; i < n; t = a[i], a[i] = a[j], a[j] = t, i++, j = (i * m) % n);
      return a;
    }
  
    long [] shuffle(long [] a) {
      Random r = new Random();
      for (int i = a.length - 1; i >= 0; i--) {
        int j = r.nextInt(a.length);
        long t = a[i]; a[i] = a[j]; a[j] = t;
      }
      return a;
    }
  
    void shuffle(Object [] a) {
      Random r = new Random();
      for (int i = a.length - 1; i >= 0; i--) {
        int j = r.nextInt(a.length);
        Object t = a[i]; a[i] = a[j]; a[j] = t;
      }
    }
  
    int [] sort(int [] a) {
      final int SHIFT = 16, MASK = (1 << SHIFT) - 1, SIZE = (1 << SHIFT) + 1;
      int n = a.length, ta [] = new int [n], ai [] = new int [SIZE];
      for (int i = 0; i < n; ai[(a[i] & MASK) + 1]++, i++);
      for (int i = 1; i < SIZE; ai[i] += ai[i - 1], i++);
      for (int i = 0; i < n; ta[ai[a[i] & MASK]++] = a[i], i++);
      int [] t = a; a = ta; ta = t;
      ai = new int [SIZE];
      for (int i = 0; i < n; ai[(a[i] >> SHIFT) + 1]++, i++);
      for (int i = 1; i < SIZE; ai[i] += ai[i - 1], i++);
      for (int i = 0; i < n; ta[ai[a[i] >> SHIFT]++] = a[i], i++);
      return ta;
    }
  
    boolean nextPerm(int [] a) {
      int j, i = a.length - 2;
      for (; (i >= 0) && (a[i] > a[i + 1]); i--);
      if (i >= 0) {
        for (j = a.length - 1; a[i] > a[j]; j--);
        a[i] ^= a[j]; a[j] ^= a[i]; a[i] ^= a[j]; // swap(a[i], a[j])
        for (int l = i + 1, r = a.length - 1; l < r; l++, r--) {
          a[l] ^= a[r]; a[r] ^= a[l]; a[l] ^= a[r]; // swap(a[r], a[l])
        }
      }
      return (i >= 0);
    }
  
    void flush() {
      pw.flush();
    }
  
    void pause() {
      flush(); System.console().readLine();
    }
  }