WRWEB ENGINEERING
REFORGING04 · 1/7
ALL LESSONS→WEEK 04→MONDAY

UNDERSTAND · COMPUTER + TERMINAL

Objects and hashes

Git as a content-addressed database30–60 MINUTESCORE + PRACTICAL
01

GROUND

Problem

What becomes confusing, fragile, or impossible without understanding objects and hashes? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Before integrated development environments, the operating system, terminal, files, and text streams were the environment. Their small composable interfaces still underpin modern tooling.
02

LEARN

Concept explanation

Objects and hashes belongs to “Git as a content-addressed database”. Git stores immutable blobs and trees; commits point to trees and parents while refs remain movable names.

For objects and hashes, trace concrete input, state transition, output, and failure through an operating-system boundary made visible through bytes, files, processes, and exit status.

Programs are processes. They read bytes, transform state, write bytes, open files and sockets, then report success or failure through observable boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Observable

Evidence produced by the objects and hashes experiment: output, state, trace, bytes, timing, or diagnostics.

Invariant

Condition that must remain true while inputs or implementation of objects and hashes change.

Boundary

Point where objects and hashes crosses ownership, representation, time, process, network, or trust.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Baseline · one variable

SETUPCommit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.

OBSERVEChanged bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.

WHY IT MATTERSThis isolates the normal contract of objects and hashes; preserve its raw evidence as the control for every later comparison.

Boundary · same contract, harder input

SETUPAmend content with same filename and compare object IDs.

OBSERVERecord what remains invariant and the first representation, owner, size, or timing value that changes in Ghostty · tmux · hx · shell · curl · Git.

WHY IT MATTERSA boundary example is useful only when one named dimension changes and everything else stays comparable.

Failure · evidence before repair

SETUPDetach HEAD, make a commit, move away, then recover it from reflog.

OBSERVECapture the first divergence from the baseline, including exact input, diagnostic, state, and recovery result. Expected recovery: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.

WHY IT MATTERSThe diagnostic is part of the interface. Repair the proven cause, not the most visible symptom.

Cross-layer · follow ownership

SETUPTrace objects and hashes one layer below its usual abstraction through an operating-system boundary made visible through bytes, files, processes, and exit status.

OBSERVEUse wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.

WHY IT MATTERSThe lower layer is earned when it explains evidence the current layer cannot. Otherwise keep objects and hashes at the simpler boundary.

03

SEE

Worked example

Start from supplied lab.sh. Focus: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.

  1. Run: chmod +x lab.sh && ./lab.sh
  2. Save baseline evidence. Use wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.
  3. Boundary case: Amend content with same filename and compare object IDs.
  4. Failure case: Detach HEAD, make a commit, move away, then recover it from reflog.
RESULT
Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves. Starter-level baseline: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
04

START HERE

Starter material

PREREQUISITESA POSIX shell with wc, od, chmod, and standard filesystem commands (macOS, Linux, or WSL).

ONE-TIME SETUPmkdir reforging-lab && cd reforging-lab

Create lab.sh, paste this exact content, then run the command below.

#!/usr/bin/env sh
set -eu
lab="${TMPDIR:-/tmp}/reforging-lab"
mkdir -p "$lab"
printf '%s\n' 'baseline: objects and hashes' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"
RUNchmod +x lab.sh && ./lab.sh

STOP / CLEANUPNo background process. Keep generated evidence files for your notebook.

05

DO WITH GUIDANCE

Guided exercise

Observe one rule: objects and hashes

  1. Normal case: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  2. Write predicted evidence from this named case before running starter.
  3. Change one input while holding environment constant.
  4. Run exact normal case. Save commands, inputs, outputs, and diagnostics in notebook.
  5. Explain changed evidence using lesson mental model in no more than five sentences.
Concrete guided solution
  1. Copy the supplied lab.sh unchanged and run: chmod +x lab.sh && ./lab.sh
  2. Write this prediction before inspecting output: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
  3. Perform only the named normal case: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  4. Save the raw output, then annotate input → transition → evidence. Use Ghostty · tmux · hx · shell · curl · Git to confirm the transition rather than inferring it.
  5. Compare prediction with evidence; if they differ, keep both and write the rule that explains the difference. Reference baseline: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
06

DO ALONE

Independent exercise

Find the boundary: objects and hashes

  1. Create second case from blank file: Amend content with same filename and compare object IDs.
  2. Then create controlled failure: Detach HEAD, make a commit, move away, then recover it from reflog.
  3. Use Ghostty · tmux · hx · shell · curl · Git to prove behavior, then repair controlled failure.
  4. Compare result against supplied acceptance checks and reference approach before marking complete.
Concrete independent solution
  1. Duplicate the starter into a clean comparison case; change only this boundary: Amend content with same filename and compare object IDs.
  2. Save its evidence beside the baseline and identify the first changed value. Use wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.
  3. Create the exact controlled failure: Detach HEAD, make a commit, move away, then recover it from reflog.
  4. Reproduce baseline exactly: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  5. Write observation table with columns input, state transition, output, and failure for objects and hashes.
  6. Run boundary case unchanged: Amend content with same filename and compare object IDs.
  7. Trigger controlled failure: Detach HEAD, make a commit, move away, then recover it from reflog.
  8. Compare evidence with reference outcome: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
  9. Rerun baseline, boundary, and repaired failure together. Accept only if all reproduce: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
07

COMPARE

Expected result

  • Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
  • Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
  • Controlled objects and hashes failure produces captured evidence; repair restores stated invariant without hiding error.
08

PROVE

Acceptance checks

Lesson is complete only when every check is true. Each check is stored locally and travels with your JSON backup.

0/5 complete · saved on this device

09

UNSTICK

Hints

Reveal hints
  1. Start with supplied normal case exactly as written: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  2. For boundary case, change only named dimension: Amend content with same filename and compare object IDs.
  3. If result is confusing, diff raw inputs and evidence before editing implementation.
  4. If tool shows nothing useful, move observation one boundary lower: representation, runtime, OS, or network.
10

VERIFY

Solution

Attempt both exercises before opening reference approach.

Reveal reference solution
  1. Run unmodified starter and preserve baseline evidence: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
  2. Reproduce baseline exactly: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  3. Write observation table with columns input, state transition, output, and failure for objects and hashes.
  4. Run boundary case unchanged: Amend content with same filename and compare object IDs.
  5. Trigger controlled failure: Detach HEAD, make a commit, move away, then recover it from reflog.
  6. Compare evidence with reference outcome: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
11

PREDICT · INSPECT · BREAK · DEBUG · MEASURE

Interrogate reality

Prediction: write expected output, state transition, ordering, and failure evidence before running either exercise.

Inspection: Use the shell, process table, filesystem metadata, curl, and Git plumbing. Never trust a command you cannot observe.

Measurement: Count bytes, processes, descriptors, syscalls, round trips, and elapsed time before explaining performance.

INSPECT

Capture raw evidence before explaining.

BREAK

Change one assumption and force controlled failure.

DEBUG

Find cause with Ghostty · tmux · hx · shell · curl · Git before editing fix.

TOOL DRILL · keyboard only · record one retrievable command or shortcut
12

MASTERY + FRONTIER + BOUNDARY

Own the knowledge

TEACH

Explain objects and hashes at beginner, intermediate, and senior depth.

REBUILD

Recreate smallest useful example from blank file without notes or AI.

RETRIEVE

Schedule recall for day 1, 7, 30, and 90.

Creative frontier lab

Try first without opening the solutions. The constraints invite invention; the reference gives one concrete direction, never the only valid answer.

Constraint inversion

Re-solve objects and hashes by removing the most convenient abstraction. solve it with POSIX files and pipes only—no editor plugin, framework, or opaque GUI.

CONSTRAINTKeep the same inputs, observable result, and failure evidence; change the means, not the contract.

ORIGINAL IDEATurn subtraction into a design tool: the missing abstraction should reveal which responsibility it used to hide.

Reveal frontier solution
  1. Freeze the contract as three fixtures: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands. / Amend content with same filename and compare object IDs. / Detach HEAD, make a commit, move away, then recover it from reflog.
  2. List every convenience used by the starter; remove the highest-level one while preserving chmod +x lab.sh && ./lab.sh.
  3. Implement the smallest replacement using solve it with POSIX files and pipes only—no editor plugin, framework, or opaque GUI.
  4. Run all fixtures and compare raw evidence. Keep the simpler version unless the removed abstraction has a demonstrated benefit.
Representation x-ray

Build an explanation artifact for objects and hashes: turn every invisible state into a diffable byte, process, descriptor, or exit-status artifact.

CONSTRAINTA peer must be able to locate the first divergence without reading implementation code.

ORIGINAL IDEATreat the explanation itself as a product: make invisible transitions visible, replayable, and diffable.

Reveal frontier solution
  1. Create one row or timestamped event for each transition in: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.
  2. For every row record input, representation, owner, operation, output, and tool evidence from Ghostty · tmux · hx · shell · curl · Git.
  3. Replay Amend content with same filename and compare object IDs.; highlight only changed rows.
  4. Replay Detach HEAD, make a commit, move away, then recover it from reflog.; stop at the first divergent row and attach its recovery action.
Adversarial remix

Combine the boundary and failure into a new user-visible scenario for objects and hashes. make a self-auditing artifact whose output includes the command and invariant that produced it.

CONSTRAINTDo not merely add more input. Invent a recovery interaction, alternate representation, or self-checking behavior.

ORIGINAL IDEAMake the system teach its own limits: the artifact should expose the invariant and offer a safe next action when it breaks.

Reveal frontier solution
  1. Combine these two pressures without changing them: Amend content with same filename and compare object IDs. AND Detach HEAD, make a commit, move away, then recover it from reflog.
  2. Name the invariant that must survive and the user-visible evidence when it cannot: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
  3. Implement this original direction: make a self-auditing artifact whose output includes the command and invariant that produced it.
  4. Demonstrate baseline, combined failure, recovery, then baseline again; save the sequence as a regression fixture.

Capability frontier

Push objects and hashes until another layer becomes justified. Record one robust technique, one contextual trade-off, and one labeled hack or historical curiosity.

CORE · PRACTICAL · CONTEXTUAL · HACK · FRAGILE · HISTORICAL · GOLF

Boundary

Shell composition becomes costly when state, error recovery, or data structure complexity dominates the pipeline.

If this vanished tomorrow…

Reproduce the useful behavior with files, text streams, process primitives, and a minimal compiled or interpreted program.

Why next layer is earned

Networking and HTTP are earned when local processes need to exchange representations across a boundary.