// https://github.com/bitcoin/bitcoin/blob/master/src/addrman.cpp
std::pair<CAddress, int64_t> AddrManImpl::Select_(bool newOnly) const
{
    AssertLockHeld(cs);

    if (vRandom.empty()) return {};

    if (newOnly && nNew == 0) return {};

    // Use a 50% chance for choosing between tried and new table entries.
    if (!newOnly &&
       (nTried > 0 && (nNew == 0 || insecure_rand.randbool() == 0))) {
        // use a tried node
        double fChanceFactor = 1.0;
        while (1) {
            // Pick a tried bucket, and an initial position in that bucket.
            int nKBucket = insecure_rand.randrange(ADDRMAN_TRIED_BUCKET_COUNT);
            int nKBucketPos = insecure_rand.randrange(ADDRMAN_BUCKET_SIZE);
            // Iterate over the positions of that bucket, starting at the initial one,
            // and looping around.
            int i;
            for (i = 0; i < ADDRMAN_BUCKET_SIZE; ++i) {
                if (vvTried[nKBucket][(nKBucketPos + i) % ADDRMAN_BUCKET_SIZE] != -1) break;
            }
            // If the bucket is entirely empty, start over with a (likely) different one.
            if (i == ADDRMAN_BUCKET_SIZE) continue;
            // Find the entry to return.
            int nId = vvTried[nKBucket][(nKBucketPos + i) % ADDRMAN_BUCKET_SIZE];
            const auto it_found{mapInfo.find(nId)};
            assert(it_found != mapInfo.end());
            const AddrInfo& info{it_found->second};
            // With probability GetChance() * fChanceFactor, return the entry.
            if (insecure_rand.randbits(30) < fChanceFactor * info.GetChance() * (1 << 30)) {
                LogPrint(BCLog::ADDRMAN, "Selected %s from tried\n", info.ToString());
                return {info, info.nLastTry};
            }
            // Otherwise start over with a (likely) different bucket, and increased chance factor.
            fChanceFactor *= 1.2;
        }
    } else {
        // use a new node
        double fChanceFactor = 1.0;
        while (1) {
            // Pick a new bucket, and an initial position in that bucket.
            int nUBucket = insecure_rand.randrange(ADDRMAN_NEW_BUCKET_COUNT);
            int nUBucketPos = insecure_rand.randrange(ADDRMAN_BUCKET_SIZE);
            // Iterate over the positions of that bucket, starting at the initial one,
            // and looping around.
            int i;
            for (i = 0; i < ADDRMAN_BUCKET_SIZE; ++i) {
                if (vvNew[nUBucket][(nUBucketPos + i) % ADDRMAN_BUCKET_SIZE] != -1) break;
            }
            // If the bucket is entirely empty, start over with a (likely) different one.
            if (i == ADDRMAN_BUCKET_SIZE) continue;
            // Find the entry to return.
            int nId = vvNew[nUBucket][(nUBucketPos + i) % ADDRMAN_BUCKET_SIZE];
            const auto it_found{mapInfo.find(nId)};
            assert(it_found != mapInfo.end());
            const AddrInfo& info{it_found->second};
            // With probability GetChance() * fChanceFactor, return the entry.
            if (insecure_rand.randbits(30) < fChanceFactor * info.GetChance() * (1 << 30)) {
                LogPrint(BCLog::ADDRMAN, "Selected %s from new\n", info.ToString());
                return {info, info.nLastTry};
            }
            // Otherwise start over with a (likely) different bucket, and increased chance factor.
            fChanceFactor *= 1.2;
        }
    }
}

// 下面以在centos上的操作为例，说明如何使用asmap工具
// 其中//表示注释， # 表示命令行提示符
// 安装Rust
# curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
# source $HOME/.cargo/env
// 显示rust的版本
# rustc --version
rustc 1.58.1 (db9d1b20b 2022-01-20)

// 安装openssl并指定OPENSSL_LIB_DIR和OPENSSL_INCLUDE_DIR
# yum isntall  openssl-devel
# vim ~/.bashrc
// 添加以下内容：
export OPENSSL_LIB_DIR=/lib64
export OPENSSL_INCLUDE_DIR=/usr/include/openssl

// 使生效
# source ~/.bashrc

// 编译bitcoin-asmap
# mkdir ~/asmap-work
# git clone https://github.com/sipa/bitcoin -b 202004_asmap_tool && cd bitcoin
# ./autogen.sh
# ./configure --disable-shared --without-gui --disable-wallet
# make
# sudo cp src/bitcoin-asmap /usr/bin/.

// 下载asmap-rs工具
# cd ~/asmap-work
# git clone https://github.com/rrybarczyk/asmap-rs && cd asmap-rs 

// 下载RIS raw data (只下载rrc00： https://www.ripe.net/analyse/internet-measurements/routing-information-service-ris/ris-raw-data)
// 下载成功后会在dump目录下生成rrc00-latest-bview.gz，该文件大小可能超过1G.
# cargo run --release download -n 0

// 生成 ASN bottleneck， 它是一串IP地址和对应的ASN编号的字符串,如：
// 193.189.95.0/24 AS34906
// 193.189.96.0/24 AS20850
// 193.189.98.0/23 AS33925
// 193.190.0.0/15 AS2611

// 读取并解压dump目录下的 MRT gz 文件, 解析AS路径, 并保存结果到bottleneck目录。
// 成功后在bottleneck目录脚下生成了 bottleneck.xxxxxxx.txt文件，该文件大小约为25M
# mkdir bottleneck
# cargo run --release find-bottleneck -d dump -o bottleneck

// 文件开始两行的数据如下：
// 120.236.88.0/22 AS9808
// 46.38.51.224/32 AS20764
// 至此我们已经通过RIW raw data生成了asmap所需的数据文件。

// 编码 ASN bottleneck，生成asmap.dat
# bitcoin-asmap encode ~/asmap-work/asmap.dat < bottleneck/bottleneck.*.txt

……
// Require outbound connections, other than feelers, to be to distinct network groups
if (!fFeeler && setConnected.count(addr.GetGroup(addrman.GetAsmap()))) {
    break;
}
……