The Blockchain Context

The Blockchain Context represents data taken from the transaction and the state of the blockchain. The data for the blockchain context is stored in the global object CONTEXT. CONTEXT, therefore, contains the main entities that you will interact with to manipulate your contract’s spending conditions.

HEIGHT

An integer representing the height of the current block is to be validated by miners. The number can be accessed using HEIGHT.

SELF

SELF represents the current eUTxO input box which holds the ErgoScript contract. The SELF box is of type Box, which contains information relevant to the specified eUTXO such as:

the value in nanoERGs; the box id; the proposition bytes of the guarding script for this eUTXO, the tokens stored in the box; and the box registers.

INPUTS

The eUTxOs are used as input boxes to be spent in the transaction. The boxes are stored in a collection and can be accessed using the object called INPUTS. The SELF box is part of this input box collection.

Data Inputs

Data inputs are input eUTXOs that are not spent in the transaction. They serve as ‘read-only’ boxes, usually holding information necessary for the spending conditions of the contract. Data inputs can be accessed using CONTEXT.dataInputs.

OUTPUTS

The output box eUTXOs will be created from the transaction. The collection can be accessed using the OUTPUTS object.

Headers

Block headers (Parts of each block that hold information representing the block) are available through the CONTEXT.headers function. The headers function returns a collection holding the last 10 block headers present before the current HEIGHT. Using this function allows your contract to gain insight into the most recent blocks that occurred before the transaction that your contract is being executed in. Each Header object in the collection returned by headers holds a variety of different information, much of this information pertains to the block miner, such as the miner’s PK, the nonce used to find the block, and the votes the miner submitted when the block was mined.

Pre-Headers

The CONTEXT.preHeader function gives your contract access to the PreHeader. The PreHeader object represents all of the information available to miners who are working to find the next block. Because each block miner must insert their own information to properly mine a block, the data contained within each PreHeader object is completely different depending on who mines the block. This means that your contract could execute differently depending on who ends up mining the block that spends your contract’s box.

Example

{
    // Focus on how we are accessing this data and using it in our contract

    // Checking if the id of our first input box is the same as the id of our output box
    val selfAtZero = SELF.id == INPUTS(0).id
    // Retrieving a long value from R4 of the first data input and adding it to our output value
    val boxAmountToAdd: Long = CONTEXT.dataInputs(0).R4[Long].get 
    val amountAddedInOutputs = OUTPUTS(0).value == SELF.value + boxAmountToAdd
    // Ensuring the height is greater than 700000
    val heightIsValid: Boolean = HEIGHT > 700000
    // Ensuring the nonce is even
    val randomValueIsEven: Boolean = CONTEXT.headers(0).powNonce(0) % 2 == 0

    // If the first condition is true, then the second condition is not checked
    // Checking if either our output box has the correct value added or the nonce is even and height is greater than 700000
    sigmaProp(amountAddedInOutputs && selfAtZero) 
            || sigmaProp(heightIsValid && randomValueIsEven)
}

The code uses various fields of the CONTEXT object to check certain conditions and produce a SigmaProp output. The code then combines these conditions in a true SigmaProp output if either amountAddedInOutputs && selfAtZero or heightIsValid && randomValueIsEven is true.