Lecture 6: Bitcoin Miner interactions and Game Theory Game Theory: P x S -> R x P P: Players S: Strategies R: Rewards Examples For the prisoner’s dilemma, tit-for-tat with some positive randomization is the best strategy. Trench soldiers in World War 2 decided to start aiming artillery at random “safe” locations instead of killing the enemy. This all happened without any communication. Mechanism design: Design the rules of the game with the outcome you want....
Lecture 5: Bitcoin mining How to mine Bitcoin Download and run Bitcoin-core to run full Bitcoin node Listen for a new transaction, assemble a pre-block Solve the puzzle (~270 attempts) Broadcast the block Profit The network runs on port 8333. Non-responding nodes are forgotten after 3 hours (5000-10000 nodes as of Sep 2016). Some seed nodes are hard-coded into the software. Zero transaction fee transactions were accepted until April 2012 before Satoshi Dice came around....
Lecture 4: Blockchains 80 bytes block consists of 32 bytes previous block hash, 32 bytes transactions Merkle tree hash, timestamp, bits, nonce, etc. Each block is <= 1MB to minimize the propagation times. Therefore, large transactions require more service fee to compensate miners to include the transaction in the block. Miner’s transaction checks ScriptSig (from spending transaction) || ScriptPubKey (from funding transaction) executes and this should produce non-empty stack. Empty stack or zero is false....
Lecture 3: Bitcoin overview There are three Bitcoin protocols Consensus Protocol - decides what the ledger is Transaction Protocol - assigns meaning to the ledger Network Protocol - the P2P protocol which decides what new should be added to the ledger Consensus Protocol Bitcoin fields (Virtual field) Hash - 4 bytes. SHA256-squared. This is not part of the block but is calculated on the fly. Version - 4 bytes. Set to 3, might never change....
Lecture 2: Creating a digital currency Desirable properties of a good digital ledger No deletion Temporal ordering Global consensus Semantic correctness Live - writable, no DOS, no censorship Attempts to create a digital currency in the increasing order of sophistication. A signing key based approach can confirm the authenticity of the transaction but cannot prevent double-spend. Append-only ledger with signing keys ensures a temporal ordering and global consensus, thus, prevents double-spending....
Lecture 1: Introduction Bitcoin is a cryptocurrency with distributed trust. The blockchain is a public append-only ledger. The append-only property is sufficient for having a currency. Hash functions: H: M -> T where |M| » |T| that is space of messages is larger than space of the hash. If H(m0) =H(m1) => collision. Hash function H is collision-resistant if it is hard to find the collision of H. For example, SHA-256 maps long strings to 256-bit hashes....
Don’t use platform fragments ( android.app.Fragment), they have been deprecated and can trigger version-specific bugs. Use the support library fragments ( android.support.v4.app.Fragment) instead. A Fragment is created explicitly via your code or recreated implicitly by the FragmentManager. The FragmentManager can only recreate a Fragment if it’s a public non-anonymous class. To test for this, rotate your screen while the Fragment is visible. FragmentTransaction#commit can fail if the activity has been destroyed....
A JPEG file can have Exif metadata which can provide the rotation/translation field information for a raw JPEG image. So, a landscape raw JPEG image could actually be a portrait because it’s EXIF orientation could be set to ORIENTATION_ROTATE_90, the best way to handle such scenarios is to either use a library like Picasso or Glide or at least learn from them. Here is a piece of code from Picasso which loads a JPEG as an in-memory bitmap and performs the right translation/rotation....
I installed Android Studio via homebrew “brew cask install android-studio” as a part of my automated Mac OS setup. Recently, Android Studio prompted me that an update is available. When I accepted to update, it failed with an error “Studio does not have write access to /private/var/folders/wt/rjv6_wcn4f97_2nth7fqftqh0000gn/T/AppTranslocation/19A80F28-865B-41FC-AA87-B8E43C826FCB/d/Android Studio.app/Contents. Please run it by a privileged user to update.” This error was confusing; I was running Android Studio as myself, a nonprivileged user and the same user owned this directory....
All languages have certain semantics for dealing with error cases. C deals with them by setting error codes. Java deals with them by throwing exceptions. JavaScript deals with them by throwing exceptions as well but unlike Java, it does have any concept of checked Exceptions. The JS interpreter just stops. And this has some interesting implications in hybrid scenarios like a Webview based app. Consider a simple Android app where most of the code is in JavaScript but is making a request to Java layer....