modify_status_on_the_fly.cpp

#include <iostream>
#include <memory>
#include <json/value.h>
#include <json/reader.h>
#include <json/writer.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <ext/stdio_filebuf.h>

#define CHILD_STDOUT_READ pipefds[0]
#define CHILD_STDOUT_WRITE pipefds[1]

static std::string change_battery_full_text(const std::string &text) {
    std::string ret;

    bool pass = true;
    for(char c : text) {
        if(c == ',')
            pass = false;

        if(c == '%')
            pass = true;

        if(pass)
            ret += c;
    }

    return ret;
}

static void err(const char *desc) {
    std::cerr << "modify_status_on_the_fly error: " << desc << " fail: "
        << strerror(errno) << "\n";

    exit(1);
}

/*
 * reads a JSON stream from i3status_out_stream, substitutes "12,34%" with "12%" in the battery section
 * in JSON and outputs all input to stdout
 */
static void main_modifier_thread(std::istream &i3status_out_stream) {
    std::string line;

    Json::StreamWriterBuilder writerBuilder;
    writerBuilder["commentStyle"] = "None";
    writerBuilder["indentation"] = ""; // The JSON document will be written in a single line
    std::unique_ptr<Json::StreamWriter> writer(writerBuilder.newStreamWriter());

    Json::CharReaderBuilder readerBuilder;
    std::unique_ptr<Json::CharReader> reader(readerBuilder.newCharReader());

    while(std::getline(i3status_out_stream, line)) {
        int front_offset = 0; // 1 if line begins in ',', as we are dealing with a JSON array stream
        if(line.size() >= 1 && line[0] == ',') {
            front_offset = 1;
        }

        Json::Value v;
        if(! reader->parse(line.c_str() + front_offset, line.c_str() + line.size(), &v, NULL) 
                || ! v.isArray()) {
            // not a message we are interested in, pass it on
            std::cout << line << std::endl; // intended flush
            continue;
        }

        for(decltype(v.size()) i = 0; i < v.size(); ++i) {
            const Json::Value &name = v[i]["name"];
            const Json::Value &full_text = v[i]["full_text"];

            if(name.isString() && name.asString() == "battery" && full_text.isString()) {
                v[i]["full_text"] = change_battery_full_text(full_text.asString());
            }
        }

        if(front_offset == 1) {
            std::cout << ',';
        }
        writer->write(v, &std::cout);
        std::cout << std::endl; // intended flush
    }
}

int main() {
    std::ios::sync_with_stdio(true);

    int pipefds[2];
    if(pipe(pipefds) == -1)
        err("pipe()");

    pid_t pid;
    switch(pid = fork()) {
        case 0:
            // child
            dup2(CHILD_STDOUT_WRITE, STDOUT_FILENO);
            close(CHILD_STDOUT_READ); // don't need this in child

            execlp("i3status", "i3status", (char *) NULL);

            close(CHILD_STDOUT_READ);
            err("execlp()");

        case -1:
            err("fork()");

        default:
            (void) 0;
            // parent
    }

    close(CHILD_STDOUT_WRITE); // we won't be writing to child's stdout

    __gnu_cxx::stdio_filebuf<char> filebuf(CHILD_STDOUT_READ, std::ios::in); // libstdc++ extension
    std::istream i3status_out_stream(&filebuf);
    
    main_modifier_thread(i3status_out_stream);

    close(CHILD_STDOUT_READ);
    return 0;
}