Kotlin

windowed and chunked

`chunked(n)` splits a list into NON-overlapping pieces of size n. `windowed(n)` slides a fixed-size window across — overlapping. Useful for moving averages, n-grams, batch APIs.

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TypeScript

range — Lazy Number Iterator

A generator-based numeric range: `[start, end)` with an optional step. Lazy so it doesn't allocate a huge array up front; consume with for…of or Array.from.

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Go

Pipeline (channel chain)

A pipeline is a chain of stages connected by channels — each stage runs in its own goroutine. Classic Go pattern for streaming transformations with backpressure built in.

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SQL

Generated / Computed Columns

A column whose value is derived from other columns and maintained automatically. PostgreSQL has STORED (persisted) and MySQL adds VIRTUAL (computed on read). Saves you from triggers for derived data.

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PHP

Unique by Callback

Like array_unique, but the uniqueness test is a callback that returns the key to dedupe by. Useful for "unique by ID" or "unique by lowercased email" across arrays of objects/rows.

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Java

Immutable Collections — List.of / Map.of

Java 9+ static factory methods return immutable collections with no nulls allowed. Concise, fast, and safer to share across threads or APIs than mutable equivalents.

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SQL

date_trunc — Bucket Times into Hours/Days/Weeks

`date_trunc('day', ts)` rounds a timestamp down to the start of the day. Use for time-series GROUP BYs — daily/weekly/hourly buckets without messy arithmetic.

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HTML

Textarea with Character Counter

Combine `maxlength` (hard cap) with a small JS-driven live counter. Browsers won't let users paste past `maxlength`, so this is purely UX feedback.

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HTML

File Upload (single, multiple, drag-and-drop)

`accept`, `multiple`, and `capture` give you a lot of polish without any JavaScript. Wrap the input in a styled label for a custom-looking drop zone.

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PHP

Chunk Array With Preserved Keys

Slice an array into chunks of a given size while preserving the original keys in each chunk. Useful for paginating ordered datasets without losing the row IDs.

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SQL

GROUP BY — Aggregation Basics

`GROUP BY` collapses rows into buckets; the SELECT list must be aggregates OR grouped columns. The most common reporting pattern in SQL.

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PHP

Detect MIME Type via Magic Bytes

Use PHP's built-in finfo (libmagic) to detect a file's true MIME type from its bytes — not from the extension, which can be lied about. Critical for validating user uploads.

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SQL

Transactions and Isolation Levels

Wrap related changes in a transaction so they commit or roll back together. Isolation levels trade consistency for concurrency — pick the weakest one that meets your correctness needs.

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PHP

Parse Query String Without Mangling

PHP's parse_str converts dots/spaces in keys to underscores. This alternative preserves them — important when parsing third-party query strings (e.g., webhook payloads).

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Kotlin

Immutable vs Mutable Collections

`listOf` / `setOf` / `mapOf` build READ-ONLY views; `mutableListOf` / `mutableSetOf` / `mutableMapOf` are mutable. Default to immutable — the call site is a contract about intent.

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Kotlin

flatMap and Flattening

`flatMap` maps each item to a collection, then concatenates. Use it when "for each X, produce N Y's, collect all Y's". `flatten()` is the no-mapping version.

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Go

Generic Result / Option Type

Go doesn't have built-in Result types, but generics make them ergonomic. Combine with `errors.Is` and you have type-safe railroaded error handling.

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Go

HTTP Middleware (Decorator Pattern)

Middleware in Go is `func(http.Handler) http.Handler`. Wrap a handler with logging, auth, recovery, CORS, or rate limiting — chain them together for a real middleware stack.

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Kotlin

Channel — Coroutine Communication

A `Channel<T>` is a coroutine-safe queue — producer side sends, consumer side receives. Bounded buffer applies backpressure. Use Flow when broadcasting; use Channel for hand-off between coroutines.

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Python

Result / Either with Generics

Encode "this might fail" in the type signature instead of throwing. Python 3.12+ generics syntax keeps it compact. Forces the caller to handle the failure branch.

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JavaScript

IndexedDB Wrapper

A Promise-based wrapper for the IndexedDB API — simplifies the verbose request/onsuccess/onerror pattern into clean async/await calls. Supports get, set, delete, and getAll operations on a named object store. Ideal for storing large client-side datasets, offline data, and blobs that exceed localStorage limits.

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PHP

JSON Lines (NDJSON) Stream Reader

Read newline-delimited JSON (one object per line) as a generator. Each yield gives you the next decoded record without holding the whole file in memory.

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TypeScript

Promise.allSettled — Typed Results

`Promise.all` rejects on the first failure; `allSettled` waits for every promise and gives you per-promise status. Pair with a type guard to split fulfilled from rejected.

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Python

unique_everseen — Order-Preserving Dedupe

Remove duplicates while preserving original order (set() loses order on collisions). Optional key function lets you dedupe by a derived value — e.g., lowercased email.

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Python

AES-GCM Encrypt / Decrypt (cryptography)

Authenticated symmetric encryption using AES-256-GCM from the `cryptography` package. Stores nonce + ciphertext + tag together as base64 so storage is a single column.

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Go

text/template — Simple Templating

Stdlib templating with `{{.Field}}` placeholders, range loops, if/else, and function pipelines. Safe alternative for non-HTML text (e.g. emails, config files).

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Java

Collectors.partitioningBy — Split by Predicate

Special case of groupingBy when the key is boolean — returns a `Map<Boolean, List<T>>` for the "true" and "false" buckets. Slightly more efficient than groupingBy.

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HTML

ARIA Live Region for Dynamic Announcements

Use `aria-live` on a container so screen readers announce its content changes (toast notifications, form errors, loading states). `polite` waits for a pause; `assertive` interrupts.

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TypeScript

JSON POST Helper

Companion to `http<T>` — POST a JSON body and parse a JSON response, both typed. Handles the boilerplate Content-Type header and JSON serialization.

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Python

group_by — Group Items by a Key

A dict-returning groupby that doesn't require sorted input. Like itertools.groupby but materializes the groups so consumers can iterate them multiple times.

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