// Copyright 2018 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library 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.
//
// The go-ethereum library 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 the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

{
    // hist is the counters of opcode bigrams
    hist: {},
    // lastOp is last operation
    lastOp: '',
    // execution depth of last op
    lastDepth: 0,
    // step is invoked for every opcode that the VM executes.
    step: function(log, db) {
        var op = log.op.toString();
        var depth = log.getDepth();
        if (depth == this.lastDepth){
            var key = this.lastOp+'-'+op;
            if (this.hist[key]){
                this.hist[key]++;
            }
            else {
                this.hist[key] = 1;
            }
        }
        this.lastOp = op;
        this.lastDepth = depth;
    },
    // fault is invoked when the actual execution of an opcode fails.
    fault: function(log, db) {},
    // result is invoked when all the opcodes have been iterated over and returns
    // the final result of the tracing.
    result: function(ctx) {
        return this.hist;
    },
}