Quan Tong
SICP Exercise 1.25: A simpler expmod?
2021-09-30
Exercise 1.25: Alyssa P. Hacker complains that we went to a lot of extra work in writing
expmod. After all, she says, since we already know how to compute exponentials, we could have simply written:1(define (expmod base exp m) 2 (remainder (fast-expt base exp) m))Is she correct? Would this procedure serve as well for our fast prime tester? Explain.
First, look at the original algorithm:
1 (define (expmod base exp m) 2 (cond ((= exp 0) 1) 3 ((even? exp) 4 (remainder 5 (square (expmod base (/ exp 2) m)) 6 m)) 7 (else 8 (remainder 9 (* base (expmod base (- exp 1) m)) 10 m))))
SICP Exercise 1.16: Iterative Exponentiation
2021-09-29
I am reading SICP.
Section 1.2.4 talks about the problem of computing the exponential of a given number.
The authors start with a recursive procedure:
1#lang sicp 2 3(define (expt b n) 4 (if (= n 0) 5 1 6 (* b (expt b (- n 1)))))
This requires O(n) steps and O(n) space.
then an iterative procedure:
1#lang sicp 2 3(define (expt b n) 4 (expt-iter b n 1)) 5 6(define (expt-iter b counter product) 7 (if (= counter 0) 8 product 9 (expt-iter b 10 (- counter 1) 11 (* b product))))
SICP Exercise 1.14: orders of growth of count-change
2021-09-28
Exercise 1.14: Draw the tree illustrating the process generated by the
count-changeprocedure of section 1.2.2 in making change for 11 cents. What are the orders of growth of the space and number of steps used by this process as the amount to be changed increases?
The count-change procedure:
1(trace-define (count-change amount) 2 (cc amount 5)) 3 4(trace-define (cc amount kinds-of-coins) 5 (cond ((= amount 0) 1) 6 ((or (< amount 0) (= kinds-of-coins 0)) 0) 7 (else (+ (cc amount 8 (- kinds-of-coins 1)) 9 (cc (- amount 10 (first-denomination kinds-of-coins)) 11 kinds-of-coins))))) 12 13(define (first-denomination kinds-of-coins) 14 (cond ((= kinds-of-coins 1) 1) 15 ((= kinds-of-coins 2) 5) 16 ((= kinds-of-coins 3) 10) 17 ((= kinds-of-coins 4) 25) 18 ((= kinds-of-coins 5) 50)))
URI network-path reference
2021-09-23
Problem
I’ve just updated the code to always prefix the post URI with a slash:
1if !strings.HasPrefix(p.URI, "/") { 2 p.URI = "/" + p.URI 3}
and I think that no need to update the HTML template:
1<a href="/{{ .URI }}">{{ .Title }}</a>
as the redundant slash will be removed. But I was wrong.
Looking at the link address, instead of seeing:
1http://localhost//2021/09/23/uri-network-path-reference
I saw:
1http://0.0.7.229/09/23/uri-network-path-reference
What’s going on?
Troubleshooting
The first question comes to my mind is what does href do if it begins with two slashes?
Side Projects
2021-09-23
- nearby-cities: Find nearby cities from a given city.
- geohash: Encode and decode Geohashes.
- tetris-tui: Play Tetris (support 2-player mode) in the terminal.
- helix-wezterm: Turning Helix into an IDE with the help of WezTerm and CLI tools.
- snippets-ls: A simple language server to just insert snippets into Helix.
- chatgpt: A simple terminal UI for ChatGPT.
- ims: I assisted my friend in automating his tasks on Chrome.
- vtv: Solve the final round of “The King of Vietnamese language” game show.
Docker Compose healthcheck
2021-09-09
The most important thing when running integration test using docker-compose is ensured that one container is started completely before others.
Sometime wait-for-it is not enough:
1 cassandra: 2 image: bitnami/cassandra:latest 3 ports: 4 - '7000:7000' 5 - '9042:9042' 6 volumes: 7 - /path/to/init-scripts:/docker-entrypoint-initdb.d 8 9 wait-for-cassandra: 10 image: willwill/wait-for-it 11 command: cassandra:9042 -t 60 12 depends_on: 13 - cassandra:
Do not use jq when working with large number
2021-09-07
We use etcd to store application configuration. On production, config is loaded from json file by using Python.
I’m wondering if we can use jq to do that. So, I tried something like this:
1host=${ETCD_HOST:-127.0.0.1} 2port=${ETCD_PORT:-2379} 3for key in $(jq -r 'keys[]' "$1"); do 4 value=$(jq -r ".$key" -c "$1") 5 ETCDCTL_API=3 etcdctl --endpoints="$host:$port" put "$key" "$value" 6done
Config is loaded into etcd but some values are not the same as in the JSON file.
What is going on?
fish shell - very high CPU usage
2021-07-30
Sometimes my Macbook is loud like a plow, and the culprit is usually docker. This time it’s different, top -u tells me that fish shell is taking up > 100% CPU.
I just switched from zsh to fish about a year ago after having problems with git completion.
My favorite feature about fish is probably autosuggestions. Normally, when I want to run a command again, I can control+R, type a few letters to select the command I want to run. With fish shell, I can type that command and then -> or control+F is fine.
Troubleshooting slow network file transfer
2021-06-25
We have a Hadoop data cluster. Recently, my colleague noticed that data transfer between servers was slow. Not all are slow; only half of them have problems.
The first tool that comes to mind is iperf3. After installing, I run iperf3 -s then iperf3 -c. While other servers have bandwidth > 900 Mbits/sec, the slow ones are just:
1[ ID] Interval Transfer Bandwidth Retr Cwnd 2[ 4] 0.00-1.00 sec 11.9 MBytes 99.4 Mbits/sec 0 170 KBytes 3[ 4] 1.00-2.00 sec 11.1 MBytes 93.3 Mbits/sec 0 204 KBytes 4[ 4] 2.00-3.00 sec 10.9 MBytes 91.7 Mbits/sec 0 204 KBytes 5[ 4] 3.00-4.00 sec 8.26 MBytes 69.3 Mbits/sec 102 154 KBytes 6[ 4] 4.00-5.00 sec 10.9 MBytes 91.8 Mbits/sec 3 157 KBytes 7[ 4] 5.00-6.00 sec 10.9 MBytes 91.7 Mbits/sec 0 165 KBytes 8[ 4] 6.00-7.00 sec 10.9 MBytes 91.7 Mbits/sec 0 168 KBytes 9[ 4] 7.00-8.00 sec 10.9 MBytes 91.7 Mbits/sec 0 173 KBytes 10[ 4] 8.00-9.00 sec 10.9 MBytes 91.7 Mbits/sec 0 173 KBytes 11[ 4] 9.00-10.00 sec 10.9 MBytes 91.7 Mbits/sec 0 173 KBytes 12- - - - - - - - - - - - - - - - - - - - - - - - - 13[ ID] Interval Transfer Bandwidth Retr 14[ 4] 0.00-10.00 sec 108 MBytes 90.4 Mbits/sec 105 sender 15[ 4] 0.00-10.00 sec 107 MBytes 89.5 Mbits/sec receiver
Automate office tasks with Python
2021-06-10
Hàng tháng bạn nhận được một file excel về danh sách D. Từ danh sách này bạn cần filter cột C theo tiêu chí T1 để lấy ra danh sách D1. Có được D1 rồi bạn Save As thành t1.xlsx. Làm xong T1, bạn tiếp tục với T2 -> t2.xlsx … Làm tiếp cho đến Tn -> tn.xlsx. Có được các danh sách t1.xlsx -> tn.xlsx này rồi, giờ bạn cần gửi mail:
- t1.xlsx: gửi cho P1, P2, P3